I'm a bit different to some of the replies - when i've sold a machine locally (different story if its hours away), i have mentioned that if anything happens with the machine in the first couple of months, i'm happy to come and help out fix the problem - parts at the buyers expense. I figure there are so few people in the hobby, if i can help a newbie out, they might become more interested and tell their mates about the hobby. In fact last weekend i went and looked at two machines (one was my old shadow, 2 owners later) nd ended up not charging anything for a few minor repairs, prob drove 50km or so and blew 3 hours. Both guys were interested and appreciative and learned how to make adjustments/minor repairs themselves. They also said they had a few mates interested in the hobby.
I've had a couple of guys really frustrating who continually ask questions, but over time, they have learned a fair bit from the advice i've given them, so good outcome in my mind. I just remind myself that this is a hobby, and the only way other people will learn is if we teach them. 100% agree with the sentiment that people should be able to search a bit themselves, but remember some of this is so new (and possibly intimidating) to them, they wouldn't know where to start searching. Some people don't know how to describe a lockdown bar, flippers, pop bumpers, ball trough - all things we take for granted.
I think there's a line somewhere. What you are doing is what should be done by a Pinball Technician / restorer / seller. You are almost a "dealer". I am put in the same category, even though I don't like it. Effectively, we are selling second hand cars. So we are obligated to provide help post sales, especially to newbies. In many cases, I receive calls from buyers unhappy with after sales support from dealers who should know better.
As a private seller, there are zero obligations. You buy a car privately and your headlights fair after a day, you cannot call the seller 3-4 times with niggling issues. A lamp fails = replace it. That's part of the fun.
The selling process / experience also dictates after sales support. The buyer clearly took up a lot of time, and basically agreed to a price and then tried to negotiate again. That is against "the spirit of the sale". We are no in Bali bartering over a T-Shirt. Once a price is agreed, that's it. The buyer has clearly taken advantage, and the "last straw" for me would be calls from another phone. That's just impolite.
If it was a NEW game, out the box, then the buyer has a right to jump up and down. But the buyer cannot call mobiles at any hour requesting 24 hour support.