I think you need to better prepare your customers. If I sold any system 7 with original boards, I would make it crystal clear to the buyer that it will need service in the near future. If the customer wasn't technical at all (and never owned a game), I would discourage them from buying a 30 year old game. If they were somewhat technical, I would make the sale and give them plenty of resources to make repairs themselves. Links to the wiki, here, RGP, parts suppliers, things like that. You don't need to put it in writing, but you do need to know the technical ability of the potential buyer if you're selling games that old. Can they use a DMM? Can they solder? If not, tell them to buy something newer.

Make sure that any noob customer completely understands that even a brand new pin can be high maintenance. Let alone a 30 year old game. Unless your markup is 2x, any 30 year old game should have a taillight warranty.

Did they call you first when the game went down? Before attempting any diagnosis or repair? If the answer to any of them is yes, I would suggest improving your final sale talk.

Sounds like at least part of it is you being too kind. Perhaps a 30 day warranty, with the selling price adjusted accordingly, would work better for you (and your customers). You could also offer warranties (1 month, 6 month) at an added cost.

Not a problem at all. You don't even need to own a game to enjoy this hobby. All it takes is a quarter or two. You're still very much in the hobby.

You don't list your location, but have you considered operating games? Operating older games is gonna be tough, but if you enjoy the tinkering, you might want to consider it. Location play is starting to pick up in many areas. It's the absolute best way you can contribute to the hobby. You won't get rich doing this either, but it pays off in other ways.