I heard recently that a man brought a silver certificate into a pawn store to sell it (it was the pawn store featured on a History Channel television show that documents its more interesting purchases). Although silver certificates no longer can be used to purchase silver, they have value as collectors' pieces, as old coins and stamps do, and the value depends on their condition and how rare the item is. The pawn store and the seller agreed to a price based on their joint belief that silver certificates from that era and in that condition are worth an approximate price, and then added to that amount money for the buyer's overhead and profit margin. When the buyer took the silver certificate to be graded and appraised, the appraiser noticed that the back side of the certificate had been printed upside down, making the certificate worth 10 times more than originally thought. Although neither the buyer nor seller were aware of the certificate's true collector's value, now that the facts are out, does the buyer owe the seller a share of his windfall?
Normally, in cases of significant underprice, the seller can demand a retraction of the sale (Shulchan Aruch, Choshen Mishpat 237:2, q.v.), though he cannot demand that the sale remain intact but the buyer pay the difference in price (S'ma :6). Although that doesn't normally apply to documents (SA :29), it does when the underprice is by more than half (Rama :29 and S'ma :52). (Anyway, I suspect they would apply to this type of document, which is valuable not as a document but as personalty, so the reason documents are excluded normally probably wouldn't apply; see e.g. Pische S'shuva :22.) Although these rules don't normally apply to antiques, they do when there is a clear discrepancy between the price paid and the market value (source). Finally, although the discussion of underpricing generally assumes the buyer did it on purpose, I see no one who says that that's a condition.
However, if this is relevant to you, then don't rely on what you read here: consult your rabbi.
Notwithstanding msh210's comment, I spoke to Rabbi Dovid Rosenbaum (YI Shomrai Emunah, Silver Spring) about this. He said that usually merchants in this field would agree with the seller that the buyer would take any risk of loss, as well as benefit from any gain in value. Thus, it is in the seller's interest to have the item appraised first by an expert, because given the conditions set for the sale, he would not be able to protest the sale to the beis din. If, however, the parties did not agree to the condition, then the halacha would be as msh210 stated. The owners of this particular pawn store normally tell sellers that they assume the risk of loss (usually as a bargaining ploy), but that type of notice would appear to be sufficient, according to Rabbi Rosenbaum.