Check it out carefully. Rot is sneaky. Coating it in fiberglass cloth is going to be very difficult because fiberglass cloth and mat don't like laying over hard corners. When people build wood boats that they cover in fiberglass they'll radius each corner and filet each inside corner so there is a nice smooth bonding surface for the fiberglass.
Coating it in resin might work, but it won't add any significant strength without the underlying fiberglass material. Also, you'll want to ask him what kind of waterproofing he's used because some will inhibit the resin's ability to adhere to the wood.
You'll want to ask the seller what kind of wood the boat was made from, what construction method he used, how old it is, was it stored in the water/outside but covered/or just laying around. If you've got any pictures of the boat it will help provide a few more answers.