I think the correct procedure to buy or sell a steem account should be through a reliable witness or escrow. Otherwise you never know the trust deficit or surplus with a third party. But a witness can perform that role who is having enough fund already invested in this system and for a mere steem account he will not do anything which will be against his hard earned reputation. Simply put, with a escrow, you can buy or sell an account very easily and in a trustworthy manner. But again you have to check what are the fees does one charge for the escrow service.
I know outside steemit also there are people buying and selling steem account. But If I will be in that position then I will simply pay 3 steem or 4 steem to blockctrades or anonsteem and will buy a steem account and will build my own account with my fresh activity. You must be wondering why I am suggesting this one. You never know what kind of behavior or impression that account holder had in steem community. Whether it was in the limelight for abusive and spam behavior and if you are buying such an account, then no matter how genuine you are, your activity may be looked upon as if you are the same old abusive person to this community.
Therefore there is always a significant risk involved in buy an old steem account. If you know properly and if you can thoroughly make a research on the past behavior of that account, that is good but time consuming too. If you are getting carried with reputation, then I will suggest with hard work & dedication you can earn a good reputation like 60 just in 3 months with out even investing. The low price of steem has this advantage now to at least gain reputation in quick time. So this is my own apprehension on this topic. Rest is up to the individual choice. Thank you.