My greatest bargains have been incomplete bikes or components. Most recent was a decent road bike with Tange prestige tubing and all Shimano 600 kit. Free due to one bad wheel. I pick up a lot of free tubular wheelsets. I personally ride these, after a careful regluing, or sometimes change out the rims for clinchers and then sell them.
Components: most of my scores have been used and incomplete. Like some relatively recent Campagnolo Record Titanium brakes. Barrel adjusters missing. And a Campagnolo Athena 11-speed rear derailleur for $5. Barrel adjuster and one of the jockeys was missing. There is a lesson here ... one missing part on a component slashes its value to close to zero. So the brakes should have sold for $200, but went at 1/10 of that. Good for me.
You need a spare parts inventory to make this all work. But this takes time and patience to collect.