Well, you can usually tell an experienced hobbyist appart from a novice, purely on how they speak to you. They tend to ask a load of questions about the conditions the fish are in, and whats it's being fed, rather than "will this fish be ok with....?" or "It eats algea doesn't it?". As above, if the person has set-up for zebs, they would almost deffinately have cloned the tanks filter from another tank, meaning I've have no issue with the sale, or they would have removed a load of fish to change the tank over for zebs...