You don't. Simple as that.

It's incredibly easy for someone to sell you an account then recover through multiple methods. You're more likely to get scammed than him being legit.

Change the password and maybe try calling or emailing steam confirming you are taking over the account?

Ninja-_-Hamster wrote I don't have a whole lot of money to buy games and the account comes with quite a few decent ones. I know that the account could get banned, but if I were to buy it, what would I do to ensure that they can't take the account from me?

You cannot ensure the security of the account as they'll still hold all of the details, payment methods used for the purchases etc. Meaning they've got more information than you about the account so they could easily contact Steam Support with the details and more than likely recover the account.

If you can't pay the prices Steam is charging for games go to somewhere like [ Register or Signin to view external links. ] for discounted Steam codes, I use this myself due to Steam's outrageous prices.

Also, [ Register or Signin to view external links. ] usually has a lot of bundles which you can pickup for really cheap which'll give you something to play until you can get the game you desire.

Steam Guard can also act as an IP grabber because it will send an email with the IP address of whoever is trying to access the account to the owner of the account. Don't enter Steam login credentials if they are given to you.