The trader sets up the product and the drum. Then he hits the drum and leaves.
Townspeople come and look at product.
If the townspeople decide to trade, they take some gold dust and leave it beside the product.
Then he hits the drum and leaves.
The trader comes back after hearing the drum being beaten. He inspects the gold and decides if it is enough for the salt. If not, he leaves and waits for the townspeople to come back to leave more gold.
But if he thinks it is a good deal, he will take the gold and leave. Like this, the traders and the townsfolk can trade but never meet in person.