Personally, I like to use one of my paintings as an avatar. I consider it an added little way of promoting my work, and it's also a kind of branding since I use the same avatar on Facebook, Twitter, and several other websites. I chose one that's eye-catching and easy to make out even when it's tiny (so I went with a butterfly, as opposed to, say, one of my landscape paintings).
I love seeing an image from a seller's shop as his or her avatar. I have often checked out Etsy shops after seeing a cool avatar in the message boards or elsewhere. My feeling is that using one of your items for your avatar makes your stuff more visible, which is good for business!
Just an idea - since you sell vintage clothing, maybe you could use a really creative photo of yourself wearing one of your items? Then you have the best of both worlds. :)