It's about having a large enough sum to go the long stretch.
And I doubt anyone is going to give you any of their secrets. In my case I know of an item that people pay millions for you can find for as low as 500.
The best I can tell you is find out what items cost and work your way up.
That's going to take work same as everything else. And it's dynamic so you have to know what's going on in the game from week to week. Just like you have to have EC you are ready to part with without a profit.
Honestly though it's fun taking a risk, it's sort of the one thing in the game that's actually player controlled so don't shy away from it.
No, it's about luck.
Your "long stretch" is entirely random. It could take 1 try, or 100. It's luck based.