Say you were at a yard sale and someone was selling old baseball cards and old comic books at a very low price. You come upon a rare card that is very valuable that is being sold for $1:00. the person does not know better, so would it be illegal to buy the valuable card for $1 without telling the person the real value?
Can the seller sue?
Of course not. That is the whole basis of capitalism. 
I shouldn’t have thought so. Haven’t you seen all those episodes of Antiques Roadshow where the person has bought something at a yard sale for $10 and comes to find it’s worth thousands? I can’t imagine all the buyers were clueless about their finds. Some were bound to know that what they had bought was collectible and worth a lot of money.
What law do you think this conduct might violate?
Been there; done that. Nobody ever took legal action.
Agree. There is nothing illegal about not informing a naive seller. It can be illegal to be the seller and withhold information, but the buyer has no such obligation. My mother is an antique dealer and they do this all the time–buy stuff at estate sales that is worth much more than they pay.
A guy paid $4 for and old painting at an antique market in Adamstown, Pennsylvania. My mother sells there. The buyer took it home and extracted an original copy of the Declaration of Independence from the frame, which was later auctioned for millions.
(My mother has a friend who claims to be the person who he bought the frame from but according to this story apparently a lot of people are making that claim. My mother’s friend said that she realized it was valuable, kept it in her car, and this guy saw it in the car, removed it, and carried it around to buy it but her daughter didn’t realize it wasn’t for sale and sold it to him.)
Not without first staging a revolution and overthrowing the capitalist system.
This is the whole basis of the primary economic system on the planet. You either don’t know, don’t care, or don’t believe the value that I place on an item. I either profit from my knowledge, or you profit from something that isn’t worth what I believed it to be.
The two big ones that I remember are:
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Fellow at a rock show saw a box of shiny stones that were about $10 apiece. He bought what turned out to be an enormously valuable star sapphire (IIRC it was valued at $20 million - something staggering anyhow). The buyer was pretty sure he knew what it was upfront and that the seller didn’t, and he didn’t feel that he should inform the seller or share the profits with him (I’m not taking a moral position on this, just repeating what I recall hearing). This happened in the early 1990s, I think that it was in Texas. Haven’t found a cite yet.
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A few years ago a kid went into a sports shop and saw a Honus Wagner card that had the wrong price on it. The store owner wasn’t there and the clerk didn’t know any better, so the kid bought the card very cheaply. The store owner demanded it back and the kid’s father refused. Don’t remember the outcome but the kid and his dad got a ration of crap over it as an example of how not to behave. Again, I can’t find a cite for this.
My recollection in the baseball card episode is that it was marked something like $400, and the clerk sold it for $4.00, thinking the zeros were cents, not dollars.
Valgard and Zenbeam, maybe this is the story you’re thinking of:
IIRC, this case was settled out of court for an undisclosed amount.
Yeah. In most cases, courts hold that one party may snap up a good deal, even if it seems too good to be true. There are some doctrines that can be exploited to soften this rule, but they aren’t used often. Here is a good collection of cases on the law of mistake in contracts. http://www.duhaime.org/contract/ca-con5.aspx
My wife and I bought a hand crocheted round table cloth for $80 in Miami Beach. Later at home we found that the price was $280. We went back to the store and the owner said that it was his mistake and wouldn’t take the difference. He said that it was a lesson to him to read the price correctly. Our gain, his loss.
The clerk probably would not have imagined someone paying $400 for a baseball card. Almost can’t blame them.
Scenario1:
Grandkid inherits Grandma’s house full of junk. Sells valuable antiques at flea market prices. Discovers after sale their true worth. Kid is out of luck.
Scenario2:
Grandkid inherits Grandma’s house full of junk. Asks antique shop owner to appraise items for sale. Owner agrees to buy all for pennies on the dollar. Discovers after sale their true worth. Kid recovers money.
That’d be the one. Memory plays funny tricks.