Now that you mention boats, I did sell my 26-ft sailboat for more than I originally paid for it (I bought it used). But it wasn’t much more, so it’s not something that sticks in my memory. Actually, the one thing that does stick in my memory was the buyer asking “why is it so cheap?” which clued me in to the fact that I’d misjudged the market, but by then it was kinda too late. I was still happy to get all my money back with a small bonus after many fine seasons of sailing. 
I bought a very nice touring ocean kayak (not a sit-on-top) with foot pedal rudder and a carbon fiber paddle. The boat was listed for $300, which made me wonder about its sea-worthiness, as you could easily get $1200 for the boat. Hell, the paddle was worth more than $300.
Turns out the guy’s wife was making him sell it. Poor guy was nearly in tears, he loved that kayak. I was about to hand him the $300 when his wife came outside. She told me if I’d pay $100 immediately I could have the boat. I gave the guy $300 and thanked him profusely. Looking at the boat’s title I discovered the first owner was a woman I knew!
(I still have the boat)
Somewhere someone has too many Iditarod Monopoly sets, someone has too few portable oxygen tanks.
Real Estate probably doesn’t count, but my mom bought a house in Denver in '75 for $34,000. Little place. One bath two bedroom.
My brother and I are selling it. Closing on the 22nd. $700,000.
Sort of a windfall -
Bought a Chevy Equinox (2014) a few years ago for my wife, paid $13k for it (salvage title - car had been stolen & insurance paid out before it was recovered). A couple years ago I totaled it, insurance paid out $17k which we put down on a 2019 Mustang with only 29k miles on it. Explanation from insurance company was the shortage of used cars on the market had driven prices way up.
I sold my copy of Fantastic Four #1 last year for several thousand dollars. I bought it at a flea market for less than cover price. It was in pretty decent shape.
I also sold a couple of old Spider Mans in really excellent shape for a hefty sum as well.
I also sold an unopened beta deck from Magic: The Gathering that was given to me as a gift, so it cost me nothing. It sold for a very high price to someone no doubt hoping that it would contain a Black Lotus card.
Aside from these recent collector’s deals, the best I ever did was when I got some custom T0-shirts printed up for a group in Grad School and actually made a profit on the deal. I was only hoping to break even.
Real Estate is specifically excluded in the OP. (But @wolfpup ’s story is an interesting exception)
This just boggles my mind.
Years ago I was at a booksellers convention and one of the many publisher giveaways I acquired was a copy of Manifold Destiny. Years later I started selling things on Etsy, mostly stuff I’d accumulated over the years that I really didn’t want. When I was checking on the possible value of some of it, I found that my copy of Manifold Destiny was a first addition, and I ended up selling it for something line $60.00. Pure Profit!
We gave a copy of that tom someone leaving our company back about thirty years ago. I can’t believe it sells for that much now. Great book.
It was a great book. Apparently when it was first published they didn’t print a lot of copies of the first edition because they didn’t think it would be as popular as it turned out to be.
Ex-pc shop owner
I used to go to electronics recycling events and approach the person in charge and offer them $20/piece for whichever computers I wanted. I would usually leave with 20-30 of them. It was not unusual for me to flip them for $100-$200.
Some literally just needed a windows reload but were otherwise fine.
Some I just carefully stripped the windows stickers for resale ($100-$150 alone)
Dead power supplies were extremely common, I could get cheap power supplies for like $15 at the time. 5-10 min fix.
Even machines that were not individually cost practical to fix were easily worth the $20 in parts. A couple sticks of RAM, a good hard drive, good power supply but dead motherboard.
I once pulled a 4GB GTX690 graphics card out of one of those recycled machines when they were $800 cards. I kept that for myself though.
Amusingly enough I later found a guy who approached me offering $50 for abandoned machines who was basically doing the same thing and reselling via craigslist. I probably sold him over a hundred machines over several years. he was always amazed that we had so many machines I didn’t think were worth fixing. ( I had already skimmed through them for any obvious hardware treasures.
Would you mind elaborating on that?
Hey, send me a PM if you notice this reply. Got a camera question for you.
Bought a sword for 180€ in a public auction.
very nice and after evaluation it was an authentic Napoleonic era of the German states: 4 500€.
Did you sell it for that, or was that an appraisal figure?
Less a thing for the last few versions of windows.
Win7 and earlier the “license key” was often a sticker on the case with a 25 digit alphanumeric code.
They had strong adhesive but with a little care you could peel them off intact with a fresh razor blade.
The code could then be applied to another case we just used clear packing tape and neatly trimmed around the sticker.
Technically this was not 100% kosher with OEM keys but I was using a legal key and was not running the same key on multiple machines simultaneously as the previous machine was scrapped.
We also gave lots of these keys away to people who needed windows reloads but the sticker was damaged/unreadable. This was extremely common on laptops. Having to replace a windows key would have made for cost impractical repair.
Later versions of windows using SLP keys in bios rendered this workaround unnecessary.
A while back, I had a bitcoin mining app on my phone (which actually paid out, albeit not very much). Ditto on a spare desktop computer.
While I suspect I paid more in electrical bills than the nominal cost): the individual transfers, of a few pennies at a time, likely add up to less than 10 bucks, 20 at most. And at the moment, it’s worth over 250 dollars.
Of course, being bitcoin, tomorrow it might be 50 bucks or 400.
I just remembered that back in the 90s I bought a set of Bose 901 speakers (with the amp) for a ridiculously low price ($300?) from a diplomat who was transferring and downsizing. I think they retailed new for about $1200 at that time. I used them for several years and then sold them for the new retail price in Africa to a local guy who owned a disco. For him it was a killer deal, as the VAT alone would have been exorbitant and the cost of the speakers - assuming he could even find a source - astronomical.
Reviving this thread: Has anyone here purchased undeliverable packages, or any similar thing, for resale? If so, what has been your experience with them?
There are oodles of unboxing videos on YouTube, and I just know that for every video that gets on there, there are 100 where they basically got garbage.