For maximum profit always sell your items as a Collection. Sure, selling them individually will get you some money, but not as much as a full Collection.
Do not sell your duplicate items; hold on to them! Having duplicate items means you're already on your way to completing another full Collection!
Loot enemy bodies often as there’s a chance you’ll find Collectibles on them - and also make sure to check drawers and cabinets of any buildings you visit.
If you want to level up your Collector Role quickly, I recommend buying the Outlaw Pass and the shovel as soon as you can. Once you reach Collector Level 5, purchase the Metal Detector. With these items you can rank up 5-10 levels in just one day!
Equip the Metal Detector every time you get on your horse to take a ride, as you never know what you'll find!
You can carry up to 10 duplicates of each item in your Collection, so don't worry about finding the same item over and over again.
Don't sell the Weekly Collection as it will remove items from your other Collections. Selling other full Collections will always pay higher than the Weekly Collection.
American Wildflowers are a great way to make a lot of money! Not only because there are so many of them on the map, but also because a lot of them are so close together, which allows you to collect them faster than anything else.