Do you have to tell the state you sold it?
Yes — and it tears off the title
Delaware's report of sale is built into the document you already have: all new titles carry a DETACHABLE report of sale at the bottom. Complete that, or the back of the registration card, and send it to the Delaware Division of Motor Vehicles, Vehicle Services Help Desk, PO Box 698, Dover DE 19903, or email it to [email protected]. Include the buyer's name and address, the date AND HOUR of the sale, and your signature.
Source: Delaware DMV, Tips for Privately Selling a Delaware Titled Vehicle, step 2