To be able to at least get a hint of whether the account was banned in the past or not, look at the post history. Usually, a ban that was caused by a spam tend to last for 14 days. If a user doesn't made any posts within that time-frame, you can at least infer that the account was banned. Well it isn't a surefire way to know what you're looking for, but that's just the idea that I have in mind that I think would be a possible way of determining whether the account was banned before or not. You need to sift pages by pages for you to get some information. It's better to be sure before buying an account. 10 minutes would probably be enough to find those because you're only finding for a "gap" in which the user hasn't posted for a period of time.