There's lots of factors you're not including with the used price though:
Bubba customization, test fired, honest wear/dirt, possible rust, might have box/docs but they'll have to look for it, etc..... and lets not forget the most valuable reason being no scary federal form to fill out! All of this and possibly more comes at a premium.
The web is filled with online sellers who will send you a thread converter adapter for $19.95. There's no reason to let the M9 x .75 threads get you bummed out.