The VCs should start lining up in ...five...four...three...
OP is familiar with the secondary marketplaces and knows if he could classify and sort well enough to make sets, then could potentially make money buying bulk and reselling sets on these marketplaces.
It could be a real business. He's already proven he has the chops to design it. If he doesn't, I don't doubt someone else will try.
Obviously Lego itself could do this but probably make more money from melting and recycling their own parts under their brand as new, like Apple does with its recyclers. Do we know if Lego is recyclable as new Lego by melting?
There was a short period that Lego allowed people to design their own sets and order them through Lego but it was so popular they had to shut it down.
People even buy new sets to sort them out just to get new bricks to combine into their own creations without having to buy them in bulk and not being able to use half of what they buy.
Could you make money just buying bulk, sorting out rare parts, and reselling those?
The price of buying a new (or used) box vs. the price of "Bricklink"ing the parts is usually pretty much in favor of the former, which makes sense since the latter involves more S&H fees.
In additions, new moulds or new colors for existing moulds come up all the time (yet new moulds are all designed in the same system, so that they increase the versatility of Lego rather than Playmobil-izing them). Therefore, advanced fans who design their own creations and buy bricks in bulk do get lots of new Lego boxes. For example the VW Beetle model, besides being really cool in itself, had a lot of azure bricks, including many shapes that had never been realised in that color. Likewise for some Architecture boxes.
 "New bricks are too specialized" is the Lego version of "HN is turning into Reddit"
There are plenty of people doing that but by hand. I figure that's about minimum wage, doing it like this should be quite a bit more lucrative.
Once Lego's patent expired it tried a fairly shady legal theory that the interconnect shapes were trademarked/trademarkable. They pursued this all the way to the Canadian Supreme Court where they lost unanimously.