Looks like they have auto-feedback set up to leave it when they receive a positive, so my best guess is (since you can either use eBay's default comments, or add/edit your own, which are then randomly applied), is that they edited one of the comments to a dot, either to try and remove it or without realising they'd saved it that way.
Either that or it's a very succinct way to say "here's your dot". 
