SLR Lens Mount Identification Guide
This page has photos of most of the major (and minor) SLR lens mounts; I find it quite useful for trying to figure out just what is what. But I'll reiterate a point made above: if the seller doesn't even know what camera the lens is for, they almost certainly can't judge its quality or usability. Look elsewhere. And any listing that includes the words "I don't know anything about cameras but..." should be skipped immediately, unless you just want something to use for target practice or a stage prop or chew toy.