I've seen some captive bred marbles, and when small, the marbling is faint. As they grow it gets darker. Your's appears to have potential. It certainly may be a marble. Either way, $200.00 isn't bad for a really nice looking motoro like this.
Here are some more pics. The flash on my camera is stuck on so it is still drowning out his dark patches. I have now got him eating shrimp so I am very happy!