Is a used car dealer required to disclose hail damage to vehicles?

It depends on--

1) Is the dealer aware of the damage? It may seem obvious, but a seller is only responsible to disclose damage or conditions of which the seller is aware.

2) How serious is the damage? A seller must disclose "material," or significant damage, of which the  seller is aware, but does not have to disclose every ding, dent or scratch. There is no hard-and-fast rule for when damage is material or not--the general idea is that if knowledge of the damage would influence the decision of the average reasonable buyer as to whether to buy or not, it may be material.

Under the laws of all states in this country, the seller of an item such as a used car is required to disclose to all potential buyers all known matters with respect to the item that would materially affect desirability or price paid by a willing buyer. If the seller of the used car knew that there was hail damage to the vehicle that was sold before the sale happened, then such should have been disclosed to the buyer before the transaction closed.