I think we're in a classic economic bubble with Pin prices. Right now, a bunch of people that casually played them in the 70s and 80s have real disposable income. They went to the local brewpub, saw the Metallica in the corner, played it a couple of times and remembered how much fun pinball was. They bought a machine because why not; then they decided to buy a couple used machines because variety is good, and they wanted the games they used to play back in the day. They pay the ebay/craigslist price because it's available and in good condition, and they have the money.

I'm speculating that a lot of these folks, in a year or two, they'll realize the games just aren't being played that much, break often, are heavy and hard to move, and take up a bunch of space. They'll want to get rid of their games because they've got their eyes on a boat, or an RV, or whatever. Then prices are going to collapse.