You can only trade to receive as much as you have storage capacity (example: You have 1 million metal and 100 crystal, but no crystal-storage; you sell metal to buy crystal, then you can only buy max. 9,900 crystal, because then the storage is full.)
It should be noted that there are three different kinds of merchants. The merchant that you need to call should be the same as the resource that you now possess and want to trade. For example, if you have deuterium to trade, then call a deuterium merchant. As soon as the trade is complete, the merchant leaves, so plan your trades well.You can also get a merchant through an expedition if you find a merchant whose cargo is stuck in a gravitation field. It doesn't clearly state that you get the merchant, but it says that the captain will trade with you exclusively. Then check the merchant link and you will have one waiting.