These aren't safe at all unless they come with the original CD keys to the account. If Valve deems the account has been hacked and or bought, they will lock it, and you will NOT get it back unless you have the physical key to prove you are the owner.
(Half-Life 1, Half-Life 2, etc)
You'll also be needing a bigger budget. Nobody is going to sell theirs for $100. $200 at least.