I don't purchase any media from there, no. But, I am in the process of the setting up an artist account there.
Quote: "What file specs should I be looking at when considering purchasing media for use in FPSC?"
Doesn't really matter, as long as you have something to convert the mesh file into a .x file for FPS Creator. For the prices they're asking on Turbosquid, I'd recommend sticking to the models that have already been mapped and include their texture file.
Quote: "I know, low poly counts right? But what is an exceptable range"
Depends, really. Personally, I try to keep static models between 0-2,000 polys; depending on the detail, and how noticable it will be to the player. I think bond1 has set a good poly budget for characters at around 4-5k. Weapons, I would try to keep under 3k.
Quote: "Any other suggested market places would be appreciated."
I'm always available for freelance work, and I don't charge an arm and a leg either. Modeling, mapping, texturing; I'll even send the models to ya already compatible with FPSC.

Email's in my profile.
I tried to persuade the mods to get a sticky up that showed all of the artists in this community available for freelance work, but it was a no go.
Kravenwolf