Personally, I loooooooveeeee avatars. When you are raiding an account with a lot of avs, it most likely has a few retireds as well. The problem for some people is that they have no idea what to look for, and often undervalue their accounts because of it.
Here is a complete list of all the valuable and popular retired avatars!
"Free Jhuidah!"
I Saved Mystery Island
Volcano Faerie
* Only 100 people have this avatar so it is indeed very rare, and probably on an active account! Be careful~
TCG Wizard
TCG - Wanna Play
BFM - Squished Spiders
Darigan Redeemed
IOM Moehog (Personal Favourite)
Coco Roll
TCG - Return of Dr. Sloth Champion
Plushie Nova
Darigan Darkling
Meridell Knight
There are a few I have not added simply because of their lack of demand. The above avatars listed are sure to entice buyers and add value to an account.