Once again, we are putting the Ancient Egyptians in a vacuum, saying that they did things this way and that. It may be true that they employed a barter system, but who's to say that they didn't barter an object for coins? It's highly probable. In fact, I wouldn't put it past them; they could have done so, only to melt down those coins to use the gold in their jewelry and other personal items. The coins that have been uncovered might have been neglected. But this is all speculation, Egyptology's middle name. As an heroic woman used to say "Speculation clears away the deadwood in the forest of destruction." (Or something like that).