They can sue you. The burden is on them to prove you cheated them, that you took advantage of them somehow. If there is any evidence that you are an experienced buyer and seller of collectible items, and art in particular, they would have a potentially-winnable case. If you are a dealer, you have a legal responsibility to deal with people honestly. You cannot take advantage of someone who doesn't know the value of what they have when you do.
If you are just an occasional buyer and flipper and just took a chance, they aren't likely to meet the burden of proof necessary to win. That said, you were foolish to go back and tell them. What was the point of that?