You need a transferable registration for the trailer, assuming a trailer is part of your deal. Some are not, and have that indicated on the face of the seller's registration. The boat trailer will need a NY inspection for transfer (lights, tires -- ask for info. at any authorized inspection site).
If it's a NY registration, it must specify "transferrable" on the face of it. If it's a non-transferrable registration (which will also be specified), she will have to get a transferrable one before the transaction (i.e., before you can register it in your name).