I didn't deal with you recently or block anything. I wasn't even online..
How can I check all the IPs accessing my account?
I believe admins (theymos) are the only one who can see IPs, I'm not sure if normal Staff/Global Mods can as well?
Regardless, it probably will not help your situation... anyone can connect through a VPN and login under a different IP, it doesn't necessarily prove anything. You could have logged in through a VPN, scammed someone, and then come back on under your normal IP and state your account was used by someone else.
IF someone else actually did login using your credentials, the solution to avoiding this is to sign messages from a known staked address when doing trades. If this was part of your standard trading practice, the person logging in as you would not have been able to sign a message as you.
EDIT: Also wanted to add here that the buyer should also request a signed message to do everything in their power to ensure that they are dealing with the same person, especially when a different address is being used to send BTC to.