"Bubble Market" is the world's most over-used and mis-applied expression.
A bubble market is a market where a bunch of speculators and johnnie-come-latelys show up and start buying something SPECIFICALLY TO MAKE A FAST PROFIT.
The tidal wave of new investors (buyers) creates a fake upswing in prices which in turn attracts more investors.
Eventually the thing runs out of steam and there is a crazy sell-off as people try to not ride the prices back down again.
Then you're back down to the pre-bubble prices.
If the wall street journal wrote a story saying, "PINBALL MACHINES ARE A RED HOT COLLECTOR MARKET", then you would see a bubble market for a while as all these rich effers swooped down to get in on the hot market.
On the other hand, if someone made an awesome video on Youtube about Cirqus Voltare and 100 people decided that they love the game and just have to have one and the price goes up by $1000, that's not a bubble market. That's just a normal uptick in collector interest, it happens all the time.
Collector markets are always heating up and slowing down.
Having a bunch of sellers fishing for a sucker with a super-high price is not a bubble, it's just how some people roll.
Pinball machines are a highly collectable item with a long history as collectables and a devoted market of hardcore collectors. There's no big sell-off coming.