If it is allowed why do we tag it?
For the same reason scammers get tagged. Sold accounts, especially high-ranked ones with green trust, can and have been used to scam people. Selling accounts also exacerbates the spam problem bitcointalk has, because people buy higher-ranked ones just so they can earn more money through signature campaigns. From what I've seen, more often than not their post output is as bad as any typical illiterate bounty spammer. It's inherently deceptive to own someone else's account, particularly if there's a trust history.
As I said recently in another thread, bitcointalk has rules but those sometimes differ from community standards, and right now it is very much frowned upon by a lot of members (myself included) to buy or sell accounts. DT members sometimes have to step in when the rules or the enforcement thereof isn't enough.
It's been suggested that buying & selling accounts be banned on the forum, but Theymos doesn't want to do that with the reason apparently being that it'll just happen off the forum. I don't agree with that, and I don't think account sellers or buyers are trustworthy (for the most part). At the very least they harm the health of the forum, and that's why I've been negging them for some time now.
