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.
This wouldn't work if the account seller was engaged in activity farming though, which I'm sure many sellers probably do. Basically this involves posting at least once in every 14 day activity period so that it becomes very easy to suddenly ramp up the activity score if the need arises. It's possible that an account that was once activity farmed could not have made any posts for weeks followed by a ton of posts made in a very short time. There is no rule against activity farming (at least not to my knowledge) although if it's detected that an account was farmed for its activity, then signature campaigns tend to look at such accounts negatively.
Okay that seems to be the case. Then why would an account get banned if its posts are very infrequent and isn't spamming? There wouldn't be enough grounds to ban a particular account if it abide by the rules of this forum, right? Also, I'm not saying that the above method that I've posted works all the time; it's just a method I thought that could possibly help one user on identifying whether an account got banned in the past or not, because you cannot ask a mod or a staff directly in the first place.