In the case of eBay stuff, you have to be careful alright, but I don't see what's wrong with asking a direct question. If you get a direct answer, you know where you stand. If you don't, take your money elsewhere. It's not the bargain basement it used to be - things are cheap for a reason. If they don't say "fully tested and working," send them a mail asking "Does it work?"
For a store like AmigaKit it's usually much better, but still, they've always been quick to reply to any questions I send them regarding technical details of their products, and overall quite helpful in that respect.