I would advise that a person consult their rabbi before taking such a job, but if you tell me that someone has done so, I'm not particularly shocked nor concerned.
One issue is a prohibition not just on eating non-kosher, but on deriving tangible benefit from it. (Which would mean you couldn't sell it.) There are very few categories of non-kosher that fall into this one, such as meat-cooked-in-milk. For stuff like pork rinds, a good Jewish boy isn't advised to get into the business of selling them, but if he got a case of them by mistake he'd be allowed to sell them -- thus, in a convenience store, most of what he's selling isn't particularly non-kosher, so not so much of an issue.
The other issue is if Jewish customers come in and try to buy something non-kosher, are you facilitating their sin. Generally speaking if it's someone who isn't keeping kosher normally, and if you don't sell him the product he can fairly easily get it at a convenience store down the block, that's not prohibited as "facilitation."