Do you have to tell the state you sold it?
No notice — the plate return is the act
There is no separate Maryland notice of sale. Returning the license plates to the MVA is what ends your insurance and registration obligations, and the plate return receipt is your proof it happened. Keep your insurance active until you receive the tag return receipt. Maryland also asks you to keep proof of the sale — copies of the completed title, front and back, and the bill of sale — because, in its words, these records protect you if questions come up later.
Source: Maryland MVA, Selling a Vehicle — seller's responsibilities