You can find good wood from rivers and lakes. They are already wet though, so you might want to let them dry abit before you sell them, weights less and cheaper to post.
Another good items to sell are aquarium plants. You just need a large tank, good light, CO2 and fertilisers. In good care some of the plants will grow 200% within a week, some more, soome less. It's not as easy as collecting wood because the plants need to be in good shape when you sell them (no algae or snails).