Female sales women / cashiers, how often do guys mistake being nice for flirting?

I've got a retail background and can give you a little insight into this. No, they're not trained, it's more of a natural response. Most girls in these positions feel somewhat safe. It's not like they're out at a bar with guys trying to pick them up or anything, they're at work. So a lot of them let their guard down and are a lot more personable. The ones you have to watch out for are sales women that are trying to get you to buy stuff. They are quite crafty in their usage of feminine wiles, and will make you think they promised much more than they intended just to get you to buy stuff. Cashiers are a different story, they don't need to sell you stuff, so they're just being friendly. You can usually ask a cashier for their number if it looks like you guys are getting along well, although some will balk at this (like some girls in any situation).