I'm pretty sure there are multiple things to be learned in this situation.
No, the seller isn't liable for any of this, unless you were sold it with a warranty. A used piece of commercial coin operated equipment sometimes breaks and need service.
Owning pinball machines is *expensive* if you can't repair and troubleshoot them yourself. That's an unfortunate fact of life. I don't know the whole story, but what I'm seeing sure looks like there was a bunch of part swapping going on that was unnecessary, and it doesn't look like much effort was made to fix the short, other than swapping out boards and hoping it was on the boards. A couple minutes with a meter should be able to tell you if a switch matrix line is still shorted to solenoid voltage!
Just so you don't feel so bad...
Early in my pinball 'career', I bought a Fish Tales from a seller in California, and had it shipped to Indiana.
It had a couple problems, and I sent the boards out to be fixed.
The boards returned, I replaced them, and played the game for awhile. Problems were solved. I was happy. After awhile, I decided to sell the FT and move on to a different game.
So, I put the game on eBay, and sold it for $1250 - which was my 'break even' point on it, after paying for the board rework, but a bit of a loss if I calculated the shipping into it.
A guy from IL bought it. Paid for it with a credit card through PayPal. Came and picked it up, about a 5 hour drive I'd guess. Played it in my garage for an hour, loved it, loaded it, left.
A few days later, he called me up, angry. Turns out, the game started having some problems. I'd stated it was as-is, no warranty, but when he explained what it was doing, it seemed like the problems were similar to the ones it had when I sent the CPU, driver and DMD controller out to be repaired. So, I felt like I had a little liability in the situation, not legally, but ethically and morally.
So, I looked around, trying to find a nearby tech, and had zero luck.
The nearest tech I could find was in Chicago, about a 3 hour round trip. I talked to him, he was good, so I paid him to go have a look. I paid him $250.
The tech called me, and said that there was a problem on the game he couldn't find, but if he swapped in his spare boardset he carried in his truck, the problem went away. Of course, the tech didn't want to leave his spare boardset on a $250 service call, so I did what I thought was best.
I called the customer, offered to send the tech back out again on my dime with a set of boards I would pull out of my Getaway. The customer said to just send him the boards, save myself the service call, and he'd install them, and return the boards out of the FT to me, so I could send them back to the guy who repaired them for me.
So, I sent him my Getaway boards. According to the buyer, they didn't fix the problem. He never returned either set of boards to me.
It cost me about $800 to replace the Getaway boards.
So, I sold the game for $1250
Paid the tech $250 of it
And sent the guy an $800 board set.
I got 200 bucks for my game after replacing the boards in my Getaway and paying the tech. That number is probably closer to $100 after considering eBay and PayPal fees.
I offered to refund the guy completely and come get the game, so long as it came back with both board sets. He declined.
For a couple years, I got periodic e-mails from the guy, telling me he'd found a new repair shop or tech, and they kept telling him the game was garbage, couldn't believe the POS seller, stuff like that.
And, after all that, the guy wanted to buy another game from me a year later.
I told him 'Sorry, I can't afford to sell you any more games'
Today, I fix 99.9% of everything on my games. Boards and all. If I can't do it, there are a select few repair guys I'd send a board to.
That was probably the worst pinball experience I ever had. I'm glad I didn't let it ruin pinball for me!