Two brothers in the Netherlands (both well over 70) bought lots of old surplus stuff when it was still cheap. The brother buying the European surplus is still buying. However, he now is buying the newer stuff coming free and after some modifications, virtually all of it is going as drivable trucks to Africa. The old stuff get shifted very slowly. The brother doing the US stuff is sitting on tons of deuces and 5-tons, all rotting away in his yard. He keeps repairing some of them or parting them and selling it piecemeal all over the developing and underdeveloped world.
It only works for them because they both have a large operation going on and bought most of their stuff when it was still relatively cheap AND they already have reliable contacts, buyers and routes how to shift the stuff. Being sure the scrap prices will always cover their back, they drift along. However, I think the collectors market is only a small part of their business.
Trying get into this surplus buying and parting out game now, I think, is like getting in on the steep increase in the share market just before it bursts. A very few sharp operators get rich quick, but most people get burned or sit on something they cannot shift for a long time. Sure, scrap prices will cover your back...as long as you only count the cash $$ in and out. Now think about the value loss of the money locked up in stuff laying around, space the stuff takes up that cannot be used differently, the aggro of trying to sell bits and pieces, time lost in all this with answering phones, negotiations, etc etc etc. I am not saying it can't be done, but it won't be that easy and at the end of they if you count all your direct and indirect costs including all your time, I am not sure the profit will be that big.
Then again, if you get a lot of fun out of it and have the space and $$ reserves and time, why not.