While I do believe that it's initially on the seller to pay the 3%, if it's negotiated up front that the buyer pays, I don't really see the problem here.
The main thing is that it gets paid.
It is on any prospective buyer to re-negotiate the terms of any deal here.
To be honest, Skinny should probably just charge a set rate for posting items for sale in the classifieds. That would make sure the site gets paid for every listing, and would cut out all the "oh, well it sold on another site" issue. That's just my opinion, though.
