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.
'03 Chevy Silverado 2500HD 4x4 6.0L Utility Truck
'99 Ford F350 4x4 7.3L Diesel 11' Dump Truck
'07 Interstate 7' x 16' Trailer
'06 Exmark TT 60" Kohler 20hp
'06 Exmark TT 60" Kawasaki 22hp
'04 Ferris 52" Kawasaki 23 hp w/ Trac Vac
'98 Exmark Metro 36" Kawasaki 14 hp
Toro 21" Proline Mowers
Shindaiwa LE261 Edger
Stihl KM110 Powerhead with attachments
Kawasaki Hedge Trimmer