Ok, so I was one of those sorting guys pre-factory set.
The local dealer bought vending cases, and "paid" four of us to assemble sets. Piecework and so much in trade per set.
I sorted differently than the other three guys, and had a ton of cards laid out on the desk in number order for groups of 10. Plus a bunch of 10 card complete stacks. Hadn't turned in any sets yet, so I was "way behind" the other sorters...
Then one day they had an order for 20 sets and offered a bonus to whoever got them together the quickest. Half an hour later I was done. Got a nice prewar card for myself with the bonus.
When I was doing Ebay I did pretty well with cheap cards. (well before the fees made it impossible. ) Once the fees got too high, I moved to bulk lots, usually 400 cards for 9.99 or $5. I'd moved along most of the thousands of duplicates I had by the time people stopped buying random boxes of junkwax commons. Including one sale that was 15,000 cards I delivered locally at some really cheap price per thousand. I think it was maybe $50 for the whole batch.
I could have done a lot more volume if I was more organized, or didn't try to give the buyers a good mix and just stuffed the first 400 I grabbed into the box.
To me organization is the key because you can be way more efficient.
Bens point about buying is also key.
There are places that will buy any cards at all, but at insanely low prices. The local guys moved and sold about 2.5 million cards to one of them for I think it was 1000....
If you want to do it, try a bit, and find an angle or way of doing things that works for you. Hopefully it works out and makes you rich.