I believe a 6.5x12 around here was like $1100 at the cheapest, without a rack, but has a 4 ft gate. I am assuming that this one has a drop gate for the mower. I think thats a good idea to offer $700 and have him deliver, but if he won't deliver, do it anyway. If the deck is in good shape and little to no rust on the frame, that sounds like a good deal on a trailer.
That mower should last you a good long time, so take good care of it, make sure you learn how to maintain it and operate it well. Hydros can be hard to learn at first, especially for a 14 y/o, so take it nice and easy.
8' x 18' Econoline Enclosed Trailer
'97 Ford F250 4x4
'00 GMC 3500 Dump
Exmark TT 60" Kawasaki 23 hp x2
Exmark Metro-36 Kawasaki 14 hp
Toro 21" Proline Mowers
Trac Vac 470 5.5 Briggs
Kawasaki, and Redmax Trimmers
Kawasaki KEL26A Edger
Shindaiwa EB8510, Kawasaki KRB700B Blowers
Kawasaki KHD600A Hedge Trimmer