As I've already understood from the comments below, the game is quite unintuitive and it's no wonder you're confused.
Actually, after receiving the items in your inventory, you have to open a window and wait for a visitor (a silhouette will appear + you will hear a bell sound), then click on the booth that says “trade”. The items that the visitor brought are on top, and the price for them is below them. To trade, you have to move items (with a total value greater than or equal to the offer) from the inventory to the cells below.