The answer is the same for every form of collector's market:
The most valuable period is what crashed up 40-50 year old remember in their youth. That translates to anything that is 20-30 years old. So peak pinball prices are for machines that were operating say 20-30 years ago. Not surprisingly that is about the DMD era.
By that, 60's/70's EMs have passed their peak prices.
As for Wobdrails, they are beyond memory of anyone who has cash to spend on a toy. They are relegated to museum pieces now. Of interest to most collectors as a piece of history, but not many desire them for home.
So the future in pinball secondhand market is the NIB games now. Buy a 15 year old pin now, and profit after about 5 to 15 years after that.
...We have a problem now: Machines built 15 years ago are poor, and were not played by many. So those machines will likely never be worth much - just as 80s pins not lost between 70s EM and 90s DMD. It's the Ghostbusters and Trons of today that you need to wait for to be about 15 years old to get at a low price, just prior to their peak.
