What you said is definitely true.
However there is a way around that.
-When you buy account try get as much information from the seller regarding account and if possible the first ever email address for that account.
-Make sure you keep pictures, photos etc of the account so you can use as proof
-Make sure you keep skype/mpgh log or screenshot to show Gamersfirst. Because according to there Policy, its against Terms of Condition to sell/buy accounts. That way the account gets banned.
(Ive had an incident where I bought an account and seller after a few months claimed it back. But luckily I had proof and G1 believed me and banned the account so at the end none of us got to have account
-Once you get your account buy something, so you can show transaction ID. Try figure out date of birth and date of registration from the forums. Ask seller for proof IP Address, like a screenshot etc
-If you use hacks, third party tools. Take a screenshot of you using it on your account, so if the seller does try claim it back, you can show that you hacked/or they hacked.
-Don't buy off people on the forums you don't know or that have less experience here. Even using a MM doesn't guarantee you will not be scammed after a few months.
LASTLY, if you don't want to fall for the trap? Easy make your own account and grind