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.
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.