That is not how it works. First of all, if they did do that, they would be using their only steam support ticket and if something happened down the line, they wouldn't be able to do anything about it. Also, you can easily contact steam support from another account and tell them that your account is hacked. Hence, you get to reset your password and the buyer is out of a 100 bucks. I also don't recommend buying steam accounts from untrusted users.
What do you mean by "if something down the line"? How do you know that I am not trusted?
I do not have to prove that you
are not trusted. You have to prove to us that you
are trusted. And by "something down the line", I mean if the account got hacked or something along those lines, they wouldn't have another steam support ticket to use.