Here's how to send contracts:
Click the two white sheet in the menu bar above your farm.
There, you can select a new contract. Enter the name of the recipient and drag the products on the trailer or the red square, respectively. A little windows appears. There you can enter the amount of products and the price per piece.
If your contract is accepted, you'll notice this by a system message. Should your partner decline the contract, you'll receive a message and the products are transferred back to your shelf. If you receive a contract, there is a big and blinking exclamation mark above the white sheets of paper.
Please stick to the current prices when pricing your goods. Contracts that contain overpriced or too cheap merchandise and are used to benefit an account can lead to the blocking of the aforementioned account.
Commerce between the servers is generally not possible. You can only send contracts to players that are on your server.