If there's one thing that I cannot stand, it's used makeup. The main reason I avoid most drugstore makeup is the fact that people do tamper with the product (especially lip and eye products), and boy is it disgusting! Some of the drugstore companies are catching on though, and they're making a majority of their products more tamper-proof.
There's less chance of buying used makeup at department stores, but it never really hurts to check. I know I do this before even purchasing, because I don't want to buy something that's been contaminated.
Like Celly Cell (and others), whenever I go poking around places like a CCO I will go for the products around the center. I also take them out of the box, and I inspect them closely before purchasing them.