The Carthaginians used to trade with ancient Africans by a method we call silent trade. Basically, the merchants would come ashore, leave the goods they wanted to sell on the beach, and return to their ships. The African natives would then come out, and pile trade goods - usually gold - on the beach in the amount they were willing to pay for the proffered goods - and then return to the forest.

The Carthaginians would then return and inspect the offer. If they thought there was enough gold there, they'd take it and leave, and the Africans would come out to take the goods they'd purchased.

If the Carthaginians felt the offer was too little, they'd return to their ships without the gold, and the Africans would either then up their offer, or just take their gold and leave.

It is their custom, on arriving amongst them, to unload their vessels, and dispose their goods along the shore. This done, they again embark, and make a great smoke from on board. The natives, seeing this, come down immediately to the shore; and placing a quantity of gold, by way of exchange for the merchandise, retire. The Carthaginians then land a second time, and if they think the gold equivalent, they take it and depart ; if not, they again go on board their vessels. The inhabitants return and add more gold, till the crews are satisfied. The whole is concluded with the strictest integrity: for neither will the one touch the gold, till they have left an adequate value in merchandise, nor will the other remove the goods till the Carthaginians have taken away the gold."

Hi there, i am taking a undergraduate level module on migration and one of the topics covered is early trade migration. My answer might not give you a complete picture to your question, but perhaps it will provide some perspective on the way trade was carried out.

As you have noted, communication was difficult in pre-modern history. I am going to quote from my source here:

"One of the oldest is Herodotus's description of silent trade somewhere on the northern or western coast of Africa. Here, and in many similar descriptions by later travelers, traders from a distance are described as bringing their goods to an accustomed place of exchange in the countryside, away from towns or villages. They deposited the goods and went away. Local traders, then appeared, deposited a quantity of their own goods and went away in their turn. When the first traders returned, they judged the value of the goods they found. If they thought an exchange was equitable, they took the new goods and left their own. If not, they adjusted the quantity of their offering and went away again, to await a silent response from their trade partners". (Herodotus, 196)

This is basically a form of silent trade. As you can see, trade did not have to take place with communication, while communication would certainly have helped. In this example, we are unsure on the duration of the whole trading process, and if there was a presence of a neutral arbiter. But it does give you an idea on how trade could have been carried out.

You can definitely read more in the source i've provided below! Cheers.

Source: Philip Curtin. Cross Cultural Trade in World History. Cambridge University Press, 1984.