I do want to purchase an account but I'm not sure how to go about it. I see a signed message saying you own the account is nice, means that it counts as proof that they changed the ownership over to you. But is there anything else or is that the only way to officially get an account? Does changing the email afterwards mean they can't get it back? It's pretty scary to buy an account even if it's only like $5. Nobody wants to pay for something and then have it taken away.
You need access to both the account and email. Then change the password, delete the old email account and use a fresh one. Then all is well.
Account buying can be risky even if you have access to email and passwords. The buyers usually sell the accounts that have red trust or loan defaulters. These things are not disclosed at the time of buying. So you should be very careful in buying the accounts.