That money appears on the next day. I haven't made the buyer NPC, but Matthew speaks about that. Everything you sold on your box is collected on the next day early and your money is added when you wake up.
On an unrelated note, I noticed you are using a log to represent vegetables in the menu, which I assume is a placeholder. Maybe you have some icons and just had not added them yet, but if you are still looking, there is a great collection of free food/veggie icons on OpenGameArt.
