As someone already mentioned, anyone can sue anyone at anytime, to answer your question. However they would not win and only lose their court fees. There was no contract because nothing changed hands. For any contract to be valid each person has to give up something. The buyer giving up their money in lieu of the purchase, even if only a deposit, creates a contract with the seller.
This is a similar scenario if you have an apartment for rent. Someone expresses interest and you say cool. However they do not give you a deposit nor sign any paperwork. Afterwards someone else comes along with the deposit, etc, so you rent to them. Can the first person sue you, yes we've already covered that, but they would lose as by not placing any deposit they did not enter into a contract with you. Simply expressing interest is not sufficient for a contract.
Walking around a dealership and expressing interest in a car does not equate to an actual purchase of that car. Literally anyone who places a watch on your item is technically expressing interest in the possibility of a future purchase. There would be far less sales, and we don't need that, if simply expressing interest of any sort makes a claim to the item and prevents it from being sold to someone else. Best not to bother replying to this person and let them move along. Best of luck to you....
