I'd keep pressing the seller. His experience with other consoles that he has sold is irrelevant. It doesn't even matter if it's a problem that he didn't experience when testing this console, bottom line is you are experiencing the issue and he should work with you to make it right.
Unfortunately, my earlier feeling that this wouldn't be a simple fix holds true. I've seen a couple people in this thread reference Maria as a possible culprit and I agree that is a logical starting point because I don't think your issue is due to the mod but rather the console. However, I've had a few boards that I've just giving up on repairing that had issues that I believe are similar to what you're describing, strange graphical issues during 7800 play. In those cases I've swapped out Maria, along with every other IC specific to 7800 play, replaced the cart connector, the 3 electrolytic caps, voltage regulator, etc... all to no avail. But now since I've received an abundance of 7800's where the shell is damaged, I have more boards than shells, so I now quickly relegate these troublesome PCB's to parts boards. I hope to eventually find the component responsible by using it for another console... no luck yet.