If you plan to start buying a lot, you MUST learn to work on these yourself. You will pay an average $125 per camera for an overhaul, and from an economic point of view, it doesn't make sense to do that. Because once you've bought eight cameras, that means you will spend another $1,000 to have them serviced.
Either that, or buy only cameras that have been serviced.
If you only expect to buy a camera here or there, then it's not as big a deal to pay someone to have them serviced.
Folding cameras generally are easy to service, and you just need some patience, some tools and a steady hand.
When you enter the world of complex cameras, such as SLRs and anything with a focal-plane shutter, then that's a different ballgame entirely.