Is it just an nt10? No Bwing. Does it come with case, parts kits, pressure tester? 400 is a good price as long as its in good shape. The nt10's weren't bad, you could get a "good" one and have no problems what so ever.
The "upgrade" from the 10 to the 11 is all in the bolt. The bolt got an upgrade to the bolt guide, and shipped with bolt spacers and flow inserts that helped "fine" tune the marker to your preference. There is a sticky all about the nt11 in here so I'm paraphrasing the updates.
From what I have seen, both work great, both are good markers as long as you maintain it after EVERY day of play and watch the pressures. Since there is no LPR it needs to be fine tuned for the solenoid and manifold to work smooth. Heck when you get the nt10 ship it off to Bwing, he does amazing work.
I made this for you!!!