How do people build large collections of bricks? How have people built up such large collections of bricks?

I would like to get into building Lego Great Ball Contraption modules, but this needs a lot of parts to play with. I've got some Lego from (30ish) years ago, but not much seems to have survived, and I've bought a couple of bug Technics sets - but I need a lot more Technics, plus more standard bricks, plates, smooth plates etc.  

Buying sets is one way, but expensive and I don't like to just make them and then instantly dismantle again.

Buying on a site like Bricklink is great for specific pieces, or when I have something to get it all in the same colour etc. 

I've seen random "1Kg lego parts" on ebay which I can try to use that to build up a load - has anyone tried this?

Or is there another way I'm missing?
I keep my eye on Craigslist and Facebook Martketplace. There’s always someone selling a collection or a bucket of pieces. There’s also a subreddit /r/legomarketplace that sometimes has good deals. I’ve had success with all of these this year and have put together a great supply of parts and sets.