Why cant you sell your gifts on FarmVille?

Most gifts can be sold for Farm Coins. If you receive something you don't need from a neighbor, you can accept the gift anyway. It will be added to your Gift Box in the game. From there, you can sell the items. Many things won't yield many coins (extra collection items, for example, only sell for 5 coins), but if you need the coins, it is worth the few extra clicks to get it and sell it.