Goes like this:
Buying:
1) use a fresh email that has never been used for steam
2) login to the account, change email first ( you get the steamgaurd code in new mail )
3) then you change pass and security question.
Selling:
1) Try to give original email or connected email with it.
1.1) With original email, change pass to something you do not use for anything else.
1.2) Set it to a fresh email for sales, as above do not use a own password.
2) let them login, you will get a code ( safe IP incase of chargebacks/scams )
3) he needs to change email first then pass/security question.
4) provide all Proof of Purchase for games after this has been done.
Hope this is what you meant?
edit: did some sales on allsteams back in the days, this is how it went then and still should be now
