According to the eBay ad it conforms to UL458. If that's true, it's definitely not junk. Each outlet is a separate circuit and there are lots of safety features involved that the casual user wouldn't even think of. If it conforms to UL458 then it is rated for marine use.
All that for $600? I don't know. If you look up Ramsond diesel gensets on Amazon, the reviews say Ramsond does not have a real address in Detroit (not there on street view). They also say many of the units are DOA and do not use the brand of parts claimed. For the amount of blatant fraud occurring, one reviewer wrote Amazon and requested they no longer carry Ramsond products, and Amazon ignored him.