Quoted from too-many-pins:
I started by buying what ever I could find cheap - fixed those up one at a time - then as I got them finished played them and decided what I liked and what should get sold. After a few years the collection was totally paid for with money from buying & selling other machines.
I never look for specific titles - I just buy stuff that is selling below current market value and keep changing out the collection as I find machines I like better than the ones we currently own. That "ride" will be coming to an end shortly after buyer & selling over 300 machines and now have about a dozen we really like in the collection. I'll still be working with parts machines but no more buying & selling machines for me after the last few are finally sold off.
When you look for specific titles it seems you tend to overpay for what you buy. When you buy with is "cheap" or what is being sold under market value you can always trade up and most of the time make a few dollars in the process. Lots of different machines will always be more fun then just picking a few in my eyes!
I did the exact thing.