After building a hearth (which will also define your settlement center – jump to it with “Return”), Traders will visit your settlements regularly.
There are three types of traders: Low, average, high. This will define the exchange rate, the amount and tier of items.
With time you will learn intricacies of trading. Buying low and selling high is standard approach of course. But here are some tips:
Observe the prices; some of them have invisible decimal digits, e.g. adding a second unit of straw will seemingly double the price of one. No such thing vice versa.
Buying raw materials rather than the end product is better, of course. Buying, for example, bones then selling them bone harpoons will make you “rich” fast. Same with slings and the early outfits (two out of one skin/leather). Shouldn’t sell flint tools, because flint is finite (but tradeable)!
Prices fluctuate depending on your era progress. Bifaces won’t be much of worth in the Iron Ages.
If the conditions of items are too low, traders won’t buy it.
Only things that are stored in buildings will be tradeable. So, don’t hesitate building them. TIP: It’s a good way to sell them soon to be spoiled stuff in exchange of “fresh” one, with a price of course. Doesn’t work for stuff which is too decayed.