Some states only require bill of sale some require notarized signature some don't require title for trailer...best way is just go to notary with the buyer and have both signatures on the titles notarized that will cover both of you. Get a copy of the notarized title for your records to prove it was signed out of your name in case buyer does something stupid with boat or trailer so you are not liable
are you an AAA member??
if so i would call your local place and talk to there title people and tell them what state that person is from..
might take them a day or two to get back to you but will have all the info you would need and then some.
would also make you look good from the buyers point of view showing you went the extra mile to get things straightened out before the deal so save you and them time an money.