I have learned a lot from my first pin purchase to now, I think that is part of the hobby or any hobby for that matter. So yes, I have certainly "overpaid" at the time of purchase on more than one pin due to hidden issues, condition, etc.. However, even when I first started, I educated myself enough to be able to understand basic value. I bought a book (Pinball Machine Care and Maintenance), read Pinwiki and of course, a ton of pinside
I should also say I don't feel bad about any of my purchases. First, Rising pin values can mask a multitude of sins, I could "cash out" at any time and make money. Second, I have had some killer deals, so Karma at work there. Third, a lot of the value of the pin is in the title - you have to understand market forces as much if not more so as condition. Finally, I do not view Pins as an investment, its a hobby. "Losing" a couple of hundred dollars when most hobbies are just a money pit anyway, for the joy and satisfaction it brings, is the cost of doing business.
So my advice is educate yourself, don't buy a $10K pin if you are not sure, start with something less expensive or go NIB with a warranty (at least you will know what your depreciation could be).