Well , no I did not know ! Actually I never thought it was a thing i would/could sell. I was browsing just the forum and I noticed this kind of activity in threads.
I was simply wondering what it was all about
There is probably not many things in this world which couldn't be sold for money. Bitcointalk accounts isn't exception. Especially considering that it can be monetized well through signature campaigns or used for scams.
You can browse through Invites & accounts section here to get image how much Bitcointalk accounts cost. Though, probably most of these users who sell accounts is scammers who don't really have accounts.