It might be Bikurriman Wold CD2, Monster Lair or Dynastic Hero (maybe even Chiki Chiki Boys). Otherwise it's a completely unrelated game or a "prototype", which is unlikely since it would've been advertised as such.
The $70 price tag is too low for a somehow undiscovered PC Engine title and is way too much for one of the likely possibilities so I'd stay away unless you get some pics first since this just sounds like a confused seller.