You're not in trouble. Just don't put it up on Ebay again.

The multi Jamma PCB's are technicaly copyright infringements as long as you don't own every original PCB on the multi PCB. They're nothing but PC's with some sort of frontend that runs mame with roms.

I can't tell you why some shops are selling them, maybe Ebay is VERY carefull with what they'll let you offer as they could get trouble, too. I guess most of the companies that hold the copyright on those 80's games don't care too much about it.