Every 6 hours the Merchant Ship refreshes 4 trades.
By trading with the Merchant Ship you can increase your trade level, also increasing the quality of the trades available.
Most trades consists out of resource cards, which you can buy by paying another resource. Other trades may contain Speed Ups.
This is a great way to turn your open resources in secure resource cards and gain speed ups.
Rarely, you will find a Black Diamond trade popping up after a completed trade.
You can lock trades to ensure they don't go away.