Big marketplaces like ebay, etsy, amazon often change the interface as part of UI testing or improving usability.
Such changes are not always beneficial. At least, even experienced users often can not figure out where they moved certain settings.
One of these hard-to-find settings is the change of user avatar, which is displayed on the reviews page and at the bottom under the description in the product listings.
So, to change the picture on this page – go to the link where the characters <YOUR_NICKNAME> replace with your nickname in the auction.
Then the right will be a link to edit.
And then – everything is simple, the avatar will have a pencil icon and you can replace the image on the other.