Check the zipper!! I think that is the biggest problem I have seen with used tents. Also, look for holes in the mesh and check the zipper. Additionally, you should check the zipper.

Actually, I'll give you a hint. I once replaced a tent because the teeth would split apart after I zipped it up. I later found out, you can fix this quite easily. Problem is caused when the part of the zipper that brings the teeth together widens from use. Take a pair of pliers and squeeze it together a little bit on both sides of the zipper and will work fine. Of course if you do this often enough, at some point in time, the zipper will break.