That will depend a lot on what's local - both with trails and with shops. If there's a popular Trek dealer near you, then you will likely see a lot of Treks for sale. If it's mainly cross country, then probably quite a few hardtails/short suspension bikes.

If you find one then just google it. See what they're selling for new and what the seller wants. Is it the right size? Does it have the componentry you're looking for? I think there's a site somewhere that's kind of like the Kelly Blue Book for bikes. You can definitely get into the intermediate or even higher end bikes with a little luck in the used market at your price point. Good luck!