The first think you should consider when buying an account is that it is ALWAYS recoverable. I honestly would recommend you building the account yourself. If you were to buy an account, make sure you buy it from highly trusted members. It may be costly, but at least you're paying for a better security than a non-trusted person with a cheaper account.
These are the only people I would possibly buy accounts from, but don't take my word for it.
Only tip you need: Buy an account from someone you can completely trust. Forget the amount you're paying for the account, imagine it's $50000. If you can trust them with that, then hopefully you can trust them to not recover. Personally, I'd just invest the money you'd use on purchasing an account on premium bot scripts and bot it up. This way it will always be yours.
Don't be fooled by all these Jagex gimmicks, like their new authenticator, JAG, and the security of the registered email. Make sure you buy it from a responsible original owner that has a high reputation and you will be fine. Of course the security features help, but never rely on them.
Basically what Sellerman said, if you feel like you should use a MM for the account trade, it's probably gonna get recovered. When buying an acc, you should be 100% fine with going first.
it's different, i want to buy an acc because on my last acc, which took me so long to train up, i wanted to bot a little, and got almost instantly banned on it..
I would just advise to only buy it off someone very highly trusted. Also, if you do buy an account make sure you perform a recovery test so you know for a fact you can or can't recovery the account if the original owner wanted to try to hack the account back.