That's how mine started getting out of control. I had one non-working machine that I rescued from a family friend's barn. My wife decided we should get a newer, working one so we would have something to play while I kept working on the first. A year later and I'm at over a dozen…
I've bought non-working pinball machines from Facebook Marketplace or Craigslist for great prices and brought them back to life. They cover a range of the pinball spectrum from electromechanical games from the 1970s through 1990s games with dot matrix displays. I fix these up, play them until I get bored of them, then sell them to make room for future projects or trade them for something new-to-me.
Separately from that, I've bought, sold, and traded to end up with a set of games that I enjoy playing. There are a couple of modern Stern games in the mix along with some mid-90s Bally/Williams games.
If I may ask, are you making a worthwhile profit on the machines you sell, or are you doing it for the love of pinball? I used to know a guy who claimed to have done this with claw machines and made a lot of money. (Yes he had a ponytail)
I'm not really making any money doing it. I learn a lot and enjoy the process.
As an example, I picked up a 1978 Bally Strikes and Spares machine from Marketplace. It'd sat in the owners garage unplayed for 10 years and no longer worked. I bought it for $400, put somewhere between 20-30 hours of work into it and around $600 in new parts. If I'm lucky, I'll be able to sell it for around $1800.
They require enough maintenance that you'll need a back up ;)