I don't understand why anyone would want to buy an old account unless the use is for sig campaign spam or some other nefarious scam.
Also you don't know the history of the account if you buy one. It could lead to issues down the road.
For example you don't know if that account has had a lot of temporary bans, and is one away from perm ban. I personally would never buy a account as you just don't know.
Or lets say an account is linked to a scam after it's sold. You don't really want your IP/name possibly mixed up in the mess.