In the above case what is the difference between buying an account with positive trust and buying trust? AFAIK the latter is not allowed.
If a transaction where an account with high positive trust (or default trust) is noticed the account should be tagged by a staff member and the account removed from default trust.
It negates the chances of either an escrow service scamming you or your trading partner scamming you because you generally can ask your trading partner to send money first to you.
Yes it negates the chance of you being scammed but allows you to freely scam others, especially if you bought the account.
They would also have to risk their initial investment in order to pull off such scam. It is also not a guarantee that such a scam will be successful and if it is not then you will have lost 100% (or close to such amount) of your investment as your positive trust account will now have negative trust.
People and especially scammers know how to calculate the risk. If you paid 1 BTC for the account just offer to sell something on the marketplace with no escrow for 2 BTC and send the guy a brick - pure profit.