There just were no intense saturated dyes in Europe back then. Murex purple was lost, indigo was a luxury import. So all dyed cloth was muddy and brownish toned by modern standards. If you wanted ruddy-brown, dark grey, olive green, yellowy-brown, then fine. But intense colors didn’t exist. Anything of any intense pure saturated color would be extremely valuable.
Except that nonelectric sewing machines are not household items for pretty much everyone in the first world.
Although the OP states that guns wouldn’t be that valuable, a good hunting rife and a scope would allow an experienced shooter to collect some cash for a limited number of clients. There would be some groups who would pay to see a king offed.
Rather than get into the chemistry of attempting to make priming compounds, I think the flintlock is the way to go if you want to play with guns.
- It is sufficiently like what they already have to avoid awkward accusations of trafficking with devils.
- It is not beyond their capability of making more after they examine yours.
To sell something for a fortune, you have to have a buyer with a fortune. That essentially means a member of the ruling class. An astute member will recognize its value immediately. If you teach them how to make corned powder, a simple improvement over the serpentine powder they use, your buyer is in a really good position to destroy his rivals on the battlefield. Give them a few quick lessons in Napoleonic era military ideas, and they now have a game breaker.
You’re right - I did take liberty in specifying a manual one. :o
Everything cost a lot in the middle ages. Clothing, as mentioned, was made by hand from the picking or shearing to the weaving and dying. (The medieval Italian village of San Gimingiano has massively tall towers that were supposedly used for hanging and drying large bolts of cloth. The cloth business supported the cost of building huge stone towers.
The same goes for everything else… mortar was made IIRC from burning limestone? Metal was melted and refined and worked - using fire. Fire usually meant someone chopping and dragging back a large supply of wood. With our casual use of massive amounts of gasoline and other energy, we forget just how laborious it was to perform any energy-intensive task. (The blast furnaces of early New Jersey, for example lasted about 5 or 10 years before they exhausted all available wood for miles around and had to be moved. ) So basically, any large lump of metal would be extremely valuable, as long as it was not some advanced alloy that could not be melted or worked in a bellows or blast furnace. Finished metal goods today are much higher quality of manufacture than in much of those days; stainless steel and nickel were very difficult to produce or work. As mentioned a knife set or something similar would be very valuable. Ditto for many other modern hand tools. Similarly, supplies like wire would be valuable as it was hard to manufacture.
Camping supplies and many other self-reliance types of products would also be very valuable. A reliable fire-started like a zippo lighter, which would go years before needing recharge, would be useful. As we saw in Back to the Future, self contained video equipment would be miraculous provided you included a self-contained charging source.
We also underestimate the amazing qualities of modern clothing. Clothes that stretch, insulated clothing that did not have the bulk of a wool fleece, quick-drying fabrics - all are miraculous items.
items like a bicycle might have some value depending on the quality of the roads. I presume many roads degenerated to mud with any significant amount of rain.
Cite?
This.
Or is a solar-powered refrigerator possible?
Reading your cite I actually came to the reverse conclusion. In my spice cabinet, I doubt that I have a pound of spices combined, so we are looking at no more than about a weeks wages. Not the sort of kings ransom you really want to get from future tech.
A few possibilities that come to mind, is a manual type writer and a ream of paper might get you far, or on a more mundane level, a few boxes of paper clips might be valuable to a jeweler given their shininess and uniformity.
My friend has an old book from around 1910 called “The Scientific American Encyclopedia of Receipts”. And that’s what it is: recipes. For everything from leather to paint to bread to an alloy of gold and aluminium that is apparently purple. We voted it “the book we most want to start a civilization with”, assuming anyone can figure out the measuring units involved. (There were a lot of “add a dash of potassium”-type steps in the recipes, too.)
I’d say that book is a good start.
The OP does say item, not items, which knocks out my encyclopedia books.
Sucks but if it has to be only one, which is it?
The problem with a big-ass nuclear submarine is that you need a big-ass crew to operate it, along with a big-ass crapload of nuclear fuel. Without that, you’ve got a neat towable fort or something - maybe you could sell it to some noble as a coastal defense fortification.
All right, I’m scrapping my sewing machine and taking a** giant roll of duct tape**. You can use it to build things, repair things, close holes in clothing, make wallets, restrain criminals, hang tapestries, shut your naggin’ wife up, and much more.
I’m amazed none of the guys here thought of it. 
Seriously, I think folks back then would pay a nice price per foot.
If we assume that guns and a reasonable quantity of ammunition are permitted (not more than an average person could reasonably carry on their person in the field), what gun or type of gun do you think would be best? I would think that a revolver could be quite hardy and last for a while, but they aren’t particularly known for having a lot of accuracy or stopping power. Assault rifle maybe, perhaps an AK-47 or something similarly renowned for reliability and serviceability? That gives you more stopping power and accuracy than a handgun but its bulkier and it still doesn’t give you a lot of long-range power. Storming a castle, hell yeah, sniping a warlord from the top of a mountain, not so much.
Any optics would also be seen as miraculous- and certainly a handheld telescope or binoculars could have many applications, especially military.
Victor Papanek’s classic book Design for the Real World, IIRC, mentions a design for a hand-cranked refrigerator. A rat-proof metal box alone would have real value, let alone a device which preserved food using only human power while not consuming valuable ice. If it made ice too - bonus.
For everyone bringing books, I wonder how readable they’d be. If we try to read English that old, it’s pretty painful work and a dictionary is often needed. I suspect someone from that age would have even more trouble reading our modern language, given how many words didn’t even exist back then. Those words won’t be listed in any dictionary, making the book itself useless without the modern time traveler taking it back.
That’s the only reason I’m not going with a good chemistry, physics or calculus textbook. The information in any of those would be priceless to the right person, but getting them to recognize the value and making it readable to them… that just doesn’t seem practical.
Also, these are pretty common items, but not exactly household items to the average person.
Thinking of my house right now… I think the most valuable items would actually be New World food items. I have potatoes and a tomato start on hand. I have corn on the cob, but I’m not sure if I could make a plantable seed out of that. Anyway, these items individually don’t have that much value, but the fact that I could plant them and then sell the food and seeds would be more valuable in the long run than any single item.
Vulcan
My candidate would be a self-winding wristwatch. Clockwork was known and valued in the middle ages, and there would be high demand for such an intricate item, especially since it would be the most accurate timepiece in the world.
Alternatively, the suggestion of an atlas should also be considered. However, it’s easier to demonstrate the value of a wristwatch than that of an atlas. We know that it’s better than any maps medieval humans had, but they wouldn’t know that. Still, a good collection of maps would have considerable value even without that demonstration, so I’m not ruling it out.
Other books would not be as immediately valuable. After all, they are written in a foreign language, Modern English, that was spoken nowhere in the world. A 14th century scholar might eventually be able to make sense of, say, the Encyclopaedia Britannica, but that would take time. A copy of the Vulgate, which is a household item in a few modern households, would have value, but not to compare with a wristwatch or atlas.
Spices, in the quantities that are found in normal households, are just not going to be in the running.
That is why they recycled damn near anything. When they dig up ancient town dumps they find little except for broken pottery.
And battlefields. Back then after a battle the locals would go over the battlefield and pick up every shoe, boot, bit of clothing, armor, weapons, buttons, you name it. Battlefields were picked clean.
In my household, they were. My mother had a Singer treadle machine before 1943, with my dad just earning grunt wages working their way out of the depression. We didn’t have a phone, but we had a sewing machine.
Back to the OP, basic tools but modern.
A hammer of today is not the same as your grandpa’s hammer. Today the metals are different. They have different balance and are designed to reduce shock. Same with hand saws, axes, and chisels. A modern shovel that can easily cut into the ground. The old ones were made of wood and broke easily.