As per the title.
I'm sorry, but whoever is doing this is a petty, small person. I have my suspicions as to who it is, but don't want to assume.
A.) There are no rules that an established member can't sell a friend's bike. At least not that I've ever noticed. I've posted many friend's bikes for sale here before.
B.) A girlfriend of a banned member isn't automatically banned from setting up accounts are they? (you guys might want to check IP's before deleting an account assuming it's the banned member)
C.) Preventing someone from being able to sell their bike because you have a personal grudge is pathetic. (oops, here I am assuming)
THIS is the bike sale that's been deleted for reference (please delete the link if it's against the "rules" of this forum). LINK (not mine).