Remember, you need items to be successful (and have fun) so pick them up as you go. I try not to leave anything behind. When I find an item my Guardian doesn’t really need, I grab it and use it to synthesize more advanced items to sell. So, at least in my opinion, it’s good to get all the items as you travel. Selling stuff is good because Gold Coins allow you to buy things from other players and get services like pets and storage space in the warehouse, which is really really nice to have. The ability to store items provides a key advantage over time.
Even things that don’t seem valuable at the moment will help you later in the game. Take Salt Peter. At first its hard to see the value of the stuff until you get enough Flax to synthesize a scroll. Then you realize that you need Salt Peter to make Gunpowder, which you need for a “Blazing Scroll”. That particular scroll gives your Guardian power enough to combat some of the game’s gnarlier varmints. I also like fireworks, for which Salt Peter is ingredient number one.