I've worked in one and I've seen the kind of stuff that goes in and sells. All I can say is that it's very gross lacking hygiene all over. They don't wash any of the bed sheets, clothes or anything like that. Just what looks good and would sell.
I LOVE shopping in those - especially if you're a broke college student like me HA!
The only thing thought... is that I do wash the clothes at least twice before I wear them - I'm paranoid that way. But thrift stores are a great place to find good quality clothes, for dirt cheap. I would recommend them highly.