You can take part in all three roles at the same time. To become a trader, all you have to do is go to the post office and pick up th he letter from Cripps. Then locate him on you map and go meet with him. It will cost you 15 gold to start it up unless you're a PS4 player. To become a collector you have to find and talk to Madam Nazar. She can be hard to find since she's in a different location every day. When you do find her, buy a collector's bag for 15 gold unless you play GTA online and collected those playing cards. I would also recommend buying a shovel from her asap so you can dig up the collectibles buried in the ground. It is $350 for the shovel.
I believe you're well over level 100 aren't you? I am as well and imo, after level 100, xp means nothing other than getting to the next treasure map level. I'd rather have the gold. I'm working on a streak in the 70's myself. Eventually it's going to have to come to an end but in the meantime I want to squeeze as much gold out of it as I possibly can.