When getting a used mattress, before putting sheets down, put down a nonpermeable mattress topper. I think it is actually worse to go to a hotel than to buy a used mattress. In the case of the hotel, you have the comforter which is never washed (and God knows what happens on it), the sheets are also used and you don't know how well they've been washed, and there isn't necessarily that mattress topper. On your own bed, you can clean it first, and then put all of your own new and clean accessories on it. Also, it is assumed that you've inspected the mattress and it looks OK before buying it. You have no idea in a hotel what that mattress looks like.
...Oh yeah, and it's the same thing for anyone living in a dorm. You are going to be sleeping on a used mattress then, also. Not a huge deal.
I wouldn't want a used mattress, but it's not the worst thing in the world.