From the prices I see here in N AL that's a good deal if everything's working properly and the engine, along with the hydro pumps and motors, are in good shape (which they should be with that few hrs on the mower). That "OBO" also let's you know there is, at least a little room for negotiation!
The best advice I could give you would be to look it over closely, and maybe give it a little test mowing some tall thick grass if possible. If you're not confident that you would catch a possible problem, maybe you know someone with a little more experience that could go with you to look at it (and maybe give it a little test run).
The mower being in warranty takes a lot of worry out of the equation on your end, but I'd still look at it very closely (and also listen very closely).
Good luck! Stan
Shut 'er down Buford....she's slangin' oil !!!!