Personally, if that Trek is in decent shape it will be worlds above the Xmart bikes. I ended up landing myself a really nice Mountain bike through Craigslist from 2003 that's aluminum frame, never had any problems.

Things to look at on the bike:

Are there any hairline cracks or major paint chipping/rust near the frame joints?

Are the shifters, derailleurs or crankset rusted?

How smooth does the ride feel?