Put back a bit of cash $500-$600. So you have a small bank of disposable cash. If you can stretch that to $1000-$1200 you are better off. Run constant ads on Craigslist "Wanted to buy Pinball Machines". Not only in your local city but cities up to 1.5 to 2 hours away. Keep the ads fresh, renew them often. WHEN someone calls you with a pin to sell. DO NOT DICK AROUND. Grab your cash and head that way immediately. No one "holds" a pinball machine until it is convenient for you to pick it up.

You need a vehicle that can haul them, a dolly and strong back because they will be in basements, etc.

You will be buying non-functional machines 95% of the time. Learn to repair them. Resell them on Craiglist for slightly more after you have repaired one. Your $300 pin can sell for $600-$700. Then like someone else said rinse and repeat.

Done right and with persistence, and hard work you will have very little perhaps nothing by flipping several machines in this manner.