Is it legal to buy and sell accounts?
Yes, buying or selling accounts is allowed, but that doesn't mean the staff or admins endorse it, but it's not against the rules.
But lenders can get easily scammed by account status of member, and scammers can purchase an account at a low price, then get a loan of, perhaps 0.5 for a senior member, then sell the negative trust account.(people are buying negative trust accounts for some reason.)
That's the lenders problem and that's why you shouldn't give collateral free loans, especially ones just based on forum rank. It's also unlikely you'll get a 0.5 loan without any though, nor will you get much for an account with negative feedback.
