I guess people are going to hope someone will type out the url for their site, as the avatar can't hyperlink to a site.
Its one type of advertising method similar to signature advertising,recent blogfeedurl spot. But less value then siganture because it doesn't have live link in it. People use it to advertise their site as image advertise. Through that you can earn few bucks for your pocket money
The soul is what counts- once you sell your soul, nothing else matters To the OP, no SEO value as such. It does create a buzz and since this forum has many webmasters logging in everyday, it serves as a fairly good way to inform the general crowd. If you had an avatar with a catchy slogan/ good picture, people will remember it. Like webtalkforums- they had avatars saying " We'll have you saying WTF in no time at all " And when you see people sporting the same avatar all round the place, it gets drilled into your head.
People sell their avatar cause they can...There is a market for it. It's better than selling your mother like some do. LOLOL
I dont know why they do it, but as long as I get cash I sell it. Get payed 5 usd for avater, I didnt have one before so I just sold it. Haha if I had one before I might not have sold. Anyhow I would never buy. Waste of cash