You can build a Trade Post facility. This enables trading and sending items to other players.
Accept a trade offer
Click on a trade offer giving you what you want. If you accept the offer partially you drag the slider to how many you want to trade. (Only full numbers of items are allowed)
Post a trade offer
The number of upgrades on your Trade Post limits how many active trade offers you may post. One less the the level of the Trade Post.
When you post and offer the number of items you want to sell is removed from your inventory.
When someone accept your trade offer you get what they pay to your inventory.
If the trade offer is not sold out after a week the remains of the offer is given back to you.
You may always cancel a trade offer and get the items for sale back.
Special trade offers
There are trade offers by players and by the system.
The system always have (bad ratio) offers to trade crystals to nearby colors, they are always there.