When I started buying the pins as a hobby, I bought barn finds, cheaper pins - some needed a little TLC, but nothing major. Slowly worked my way up to newer titles. Mostly selling more than trading, but with the intent to replace. Never a flipper, but mostly made just a little bit here and there with each deal (always sold for fair prices) and got something better. Had a local friend heavily in the trading/selling side and sometimes we'd work together for combo deals (i.e. he'd sell mine and his to get a better deal on a couple of different pins). He had connections, I'm an electrical engineer and could repair - so we benefited each other. Fun times.
Fast forward to a couple of years ago: I retired. Goal over the years was to build up the collection to the point it was mostly 'self sustaining' (i.e. own a newer title that I could trade for other newer titles, so any +/- cash outlay was minimal or zip). Likely mostly dumb luck, but I seemed to have gotten there.
I don't think I could repeat that, given today's prices. One barn find was a EM Freedom that was rather minty (needed just a little electrical work) for I think was few hundred dollars - and that had sat on Craigslist for quite some time before I decided to go look at it. Times have definitely changed.
But overall, I'm content with the lineup (Mando might go up for trade in the near future). One thing I wanted to do during retirement was keep them longer and learn the rules better (mostly still a fail at that). Plus I've gotten more picky over the years, not as many to choose from compared to when I started and everything was new to own for me. Wizard's World Pinball is not far away from me, so I can always play ~150 other pins when I want a change.