"If you were a merchant PJF, I wonder where you would take your inspiration from?"

I would never take someone else's inspiration for commercial gain without permission.

"DUO saw a shape in an illustration and got inspired to model something in the same vane."

No, duo saw an illustration and closely copied an item in that illustration in order to sell it. The shape and apparent details are quite closely duplicated (they are quite clear on the cover of Fred Gambino's book "Ground Zero").

The scenario you suggest is what Kromekat did for his "Foss" inspired tutorial for "Imagine FX" magazine. He developed his own design; there isn't a Chris Foss illustration with a ship that looks like that.

I have no idea if your ray gun models are originals done in the same vein as old style SF designs, or if they are close copies of originals by SF artists and designers working at that time. If the former, then I wish you the best of success.

As I said before, Renderosity doesn't care much about this issue (one of the reasons I don't have a gallery here). Plagiarism isn't necessarily illegal, but it is always plagiarism.