Donut gasket. A $7.00 part. Not something I have fouled up on either of the boxers I have owned, but I can see how you could knock it loose during cover installation.
I cannot tell if I am seeing messed up paint, or optical distortion due to oil on that lower valve cover area. Don't clean it all up; let the shop see it in failure mode.
I kind of look at this the same way I look at it when someone fails to notice that the old spin-on oil filter left its o-ring behind on the engine when it was removed and the new filter is installed on top of it.
Those end up leaking too. Stupid mistake, but people manage to make it. In other words, I see your issue as human-based, not machine-based, so throwing up your hands and running away seems kind of like an uncontrolled emotional outburst. I assume that you looked the bike over pretty carefully and considered its condition, and mileage, etc., and made a deal on it that you were happy with. If that was the case, I would let the dealership do their thing, and if they put the bike back into proper order, not worry about it.