This mostly happens when you buy an account whose IP is far from yours(another country).
Log it in in one IP/device only. If you try to log it in in multiple devices in a short period of time sequentially with different IPs it'll trigger a flag.
Once you log it in, your logs won't match at all so AE is forced to assume the account is either hacked/compromised and will ban it until confirmed.
You have to make your seller e-mail AE that he/she is well.. visiting/moving to another country or something like that, just let him do the talking and it'll be fine.