Farmers can place an order for 3 plots worth of crops to be planted by neighbors (depending on what they are growing). They will receive Mastery bonus, XP and Coins when their order is filled. The rewards are equal to what the crop would yield if a farmer had planted it himself. So, if you order Strawberry for 30 coins, you get 3 XP, 3 Mastery points and 75 coins when the neighbor fills your order. That neighbor in turn receives the same. A farmer can take up to 50 orders, every 12 hours.
In essence, it is as if you get 3 extra plots. While that may sound like a small number, keep in mind that you get them at all neighbors who have a Trading Post!
The Trading Post does not require or affect Bushels in any way, and can be placed both on the Home and English Countryside Farm.