Sure, trading at your own risk. And using an escrow can avoid that risk even, at least when the escrow is changing the pass, so has to login.
Though for the unescrowed trades or trades where the escrow is not logging in, it would mean supporting scammers. Since showing a ban has no disadvantage for everyone else though has the advantage that others can't be scammd so easily.
Well, when I think over it it might be rather rare cases when someone trades an account without escrow and even would know the account name before buying.
arbitrage
So there is no way to know is it scam or not?!
Mostly there are entries from campaign managers in trust ratings. Or one could check out google search with:
site:bitcointalk.org username banned
probably they assuming this account will be used for scams, so this way they will discourage sells..
But this is still legal..Why?
They forbid it once but the problem was that it did not change anything. Trades still happened, only hidden. The downside was that they did not see it happen. It's better the trades happen over forum pm.
Decoded
Simply just look in the meta thread, and usually the butthurt guy will come crying.
Though an account owner who get permabanned and has a scammy character would not do this. He would try to approach interested users with this irresistable offer.
Though like I wrote above. I think the cases might be very rare to be the reason for a change.