Marty is "spot on" - the term "backyarder" is often used to describe restorers and technicians who don't work for commercial resellers. In most cases, "backyarders" do a much better job and often set the standard that the commercial guys envy. "Backyarders" is usually just another name for a "Hobbyist" or "enthusiast".
Case in point is Marty's example - a Dealer Simpsons for $3500 VS a restored "backyarder" Simpsons for $2200. I've seen Marty's work and it is better than any dealer could possibly achieve.
Buying a reconditioned Pinball is exactly the same as buying a second hand car. Do your homework and ask heaps of questions.
If you see wear on a DE Simpsons, chances are the game was used heavily in its route days as wear on Data Easts is not normal. Sounds like it may need playfield touch ups which is expensive and time consuming.
Best thing to do is work on a budget, go have a look at what ALL the dealers have in their inventory and make an informed decision. PLAY each machine and look for broken ramps, bad touch up work, all toys present & working, all lamps and rubbers replaced, flipper kit installed, cabinet clean and that the game has the correct legs.
Have a look in the "Restoration" section. Plenty of games that "backyarders" have restored or reconditioned !