After reading the OP's story, I had some thoughts. I am not defending the seller in the least. But before everyone brings the pitchforks, I think there were a few complicating issues on this sale.
1) The buyer bought it sight unseen. That is very very risky unless you personally know the seller. I'm amazed he had so many GOOD deals!
2) After purchasing, the game sat in storage for 6 weeks before the buyer picked it up, used it and determined it was not right. How long should a buyer guarantee a used game after sale? If you buy a used car and sit it in storage for 6 weeks and then tell the dealer the transmission doesn't shift right, what do you think he is going to say?
3) Depending on the price of the game and the conditions under which it was sold, a couple of diodes on the flippers is pretty minor. I have had much worse on games that were supposedly in great shape!
Again, not defending the guy. Don't even know who he is. Just saying there may or may not be two sides to this story. If the buyer had a soldering iron and a bit of technical knowledge, he could have fixed this in a minute for 50 cents. If he lacks the basic tools and knowledge, I would suggest he cultivate a relationship with a local dealer and only purchase games that are restored, guaranteed, and delivered.

Pins can play ok for some time without flipper diodes. And then unexpectedly give problems. Missing diodes can be overlooked by someone who is not a well experienced tech, or they can break or fall off when being transported. The seller might be dishonest, he might not be a very good tech, or he honestly might not have seen any issues while the game was in his possession. He might have only played two games on it before pronouncing it 100% working. And it might have been 100% working, for those two games.
Bottom line, as I and others have said, don't buy sight unseen, play it immediately upon receipt if you do get it shipped from someone else, make any faults known promptly to the seller, not weeks afterward, and if you are not willing to pay retail, learn how to make your own minor repairs or realize you're going to have to pay your local tech to straighten out whatever you buy.