People do buy used make-up - look at the MAC auctions on eBay. I wouldn't do it with lipstick or mascara, but if you use a brush or a pump then it isn't that unhygenic. I've definitely sold used products on eBay and been very clear that they were.
I've taken mascara back, but only because there was obviously something wrong with it (it was supposed to be waterproof but didn't even stay on being worn for a couple of hours in dry conditions - it was dreadful. I think there must have been something wrong with it). Personally I wouldn't take make up back just because I didn't like it.