One more question, how can activity be more than posts count? Couldn't understand!
The accounts that are sold with a low price are usually not of good quality, have scammed before, have tried to or have some unpaid loans which you will find some time after you buy them. I know the price of an account but I have only quality accounts which are guaranteed to be accepted in signature campaigns that is why my prices differ from other sellers.
Activity can be more than post count when the account has been inactive for some time. Even this account of mine here if I continue posting can be Hero in just one month.