First off, let's not all lose our shit over a chipped back panel. You don't play the cabinet anyway. You got a HOU Pinbot. Play it and be happy. You won. Dare I say most Pinbots are in worse condition.
I'm not excusing Mayflower/STI, but you should get home and take a close look for yourself before slamming them. If the damage extends back up under the leg, and the leg is not damaged in any way, and there are no fragments from the broken wood, then (and only then) would I start to suspect that this might be previous damage, undisclosed by the seller. Some sellers, especially non-pinheads, couldn't care less about dents and dings in the cabinet. And they won't even think to mention them. Now, if Mayflower/STI did screw it up, then MAKE THEM PAY for the cost of the repairs.
Just my 2 cents.
Best regards and congratulations on your new (to you) game.
Tom