I don't think any pet store employee would want to lie about the sex of an animal. People make mistakes, especially if you see thirty people in a day who all want their hamster or whatever sexed, you're bound to make the occasional mistake.

Rabbits in particular can be difficult to sex before they mature, as well as dwarf hamsters, especially whe you're wearing awkward gloves...

We accidentally sold a very well-endowed male guinea pig as a female. *OOPS* Fortunately, everyone who buys an animal from us has to sign a paper that says they accept that we don't guarantee sex or disposition. ^_^ No money back for them! (She didn't deserve it anyways, the woman was a *****)