Q. I bought a painting at a public auction for a very high price. Can I claim onaah afterwards?
A: Some poskim maintain that there is onaah in a public auction, even if there is a common practice otherwise, since even an explicit condition that there should not be onaah is not valid if the amount of onaah is not specified (Teshuras Shai 1:456; C.M. 227:21).
However, many poskim rule that there is no onaah in a public auction for various reasons, among them minhag hamedinah, especially if non-Jews are also bidding and there is no onaah according to dina d’malchusa (Sho’el u’Meishiv 4:3:137; Mishpat Shalom 227:15).
Nonetheless, some poskim rule that if the auction was based on an appraisal that proved to be mistaken, there is an onaah claim (Nesivos 109:5).
Regardless, for items that do not have a defined market value, such as Judaica, works of art, etc., whose value is determined through the auction – there is no onaah, since their inherent value rises through the bidding (Pis’chei Choshen, Geneivah 10:15; Hilchos Mishpat, Onaah 227:25).
