HackForums stores your IP. His IP will be the same as his old accounts and he will be banned for multiaccounting because his IP will match banned accounts. HF also BLOCKS all VPN/Proxies/VPS's.
The above is correct - if you already logged in with your normal IP and got banned, your IP will be blacklisted and any accounts you make again will be banned. Though, if you're willing to spend money to buy an account, you probably already know this and have a method to access hackforums.
Also if you are to trade, it would be advisable to send the account details via PM instead of publicly because as shown above, if the account becomes recognisable it will lose its rep and will be banned for being bought. The money will go to waste.