1. Get hold of rare material.
2. Advertise it as "for trade" in as many MJ communities as possible.
3. Trade it for other rare material, and then repeat from step 1.
At least that's what I assume. I wouldn't know, because I never managed to get past step 1.
I think 99% of people can't complete the first step.
I've searched years for rare material, in every way I know possible (video material). I've had a couple of opportunities to get things, but haven't been willing to pay a big price.
I think that's the most important thing.
1) Have lots of money
2) Be willing to spend it
You gotta take a big plunge to start off with, I think. You may end up paying 200% of what the material is worth, but at least you are on the ladder. From then onwards, you can be more smart about the prices that you pay. But I think you would have to take a huge plunge and shell out a lot of money to even get "in the game".