I know people have been having trouble selling their items back to the shop, so I wrote a simple guide to explain it.
1. Go to
Shop
2. Choose the category in which the item you want to sell back is
3. Click "Sell back to shop" at the bottom of the page
4. Page will refresh, and tell you how many of which item you own
5. Enter the quantity of the item you want to sell back
6. Click "Buy" (seems strange, but it's right)
7. Click "Checkout" in the page that loads
8. Click "Submit order"
9. You're done
