1) first you should check the account's quality. you can use this for an estimate
https://bitcointalk.org/index.php?topic=1142314.02) check if the password was reset recently (sign of being possibly hacked):
https://bitcointalk.org/seclog.php3) use 1 to find accounts bitcoin address. it gives all the bitcoin addrress mentions, check all of them to see which one was his.
4) ask for a signed message from the oldest one
5) use google to find out if he was banned/has a loan/scammed/...
6) use escrow
Usually you will not get chances to review all these before paying for the account. But escrow will be able to do. So, we need to give list of check-list to escrow as well as to the seller in PM.
Other than above list I concern some of the following issues...
1. Account is a known alt account ?
2. Any negative feedback from untrusted members ?
3. Got banned in any signature campaign ?