I don’t think that Graeber is implying that what you are calling trade by “barter” never happened. I think he is simply suggesting that it did not happen as if it was the main point of the interaction. Shells from seacoasts got passed from person to person inland, yes. Certian kinds of useful materials, like obsidian, got passed from person to person outward from the sites where it could be found in abundance to the places where it was scarce, yes.
You say it must have been done by “trade” because there were language barriers so simple indebtedness would not work. You see trade as the purposeful exchange of goods between groups for their mutual benefit, and I can understand that perspective. But it is not what we actually observe in interactions between groups of people, in non-market societies, who are speaking different languages.
Let me explain it among hunter-gatherers. I spent some years studying the economy of South Eastern Kalahari hunter-gatherers — groups speaking languages like G/wi, !Xo, //ana, and Kua. These are mutually unintelligiable. I observed what happened when camping parties from different language groups came together.
For one thing, I saw multi-lingualism. Most adults were able to speak at least two other languages.
For another, I saw evidence of personal friendships being renewed. These were people who had met in the past, even if it was not for long or more than a few times a year, and had become friends after sharing some time together. Some of the people I asked about this told me they had met as children; they had known one another all their lives. So they looked forward to times of year when they camped close to the same water point, to literally keep in touch. All around me, I saw people picking up conversations, sharing the events of their lives since the last time they had met.
For yet another, I saw evidence of intermarriages. Even a single intermarriage five generations in the past gave rise to abundant kinship links bewtween these different groups.
I saw complete absence of barter: indeed, hardly any gifts even got given. What did happen was talk, and parties. Back and forth socializing where people told each other stories and jokes, demonstrated favourite dances, and shared food, especially meat. Only the people who were comfortable — because they had personal kinship or friendship ties that crossed the language/group affiliation — passed on small things like jewellery, often suggesting it was a token to be eventually passed on to another party, who was not present, but who might be seen by the reciever in future. Women especially asked each other to pass on news and “longing hearts” to absent people.
They did not WANT, it appeared to me, to create any sense of debt that even a small gift might incur. If a gift was offered between people who were not in an established friendship and/or kinship network, it was cause for distrust and even anger. The implication of such a thing was that the giver was trying to create an intimacy that had not been earned: an intimacy of indebtedness. It implied too that the meeting of the the two groups was about getting certain objects, not about reaffirming cordial relationships. Which in turn implied that it was the objects that were important and the relationships were just a means to that end. And no one wanted that. Such an implication would sever friendships and destroy the great unspoken assumption that governed their interactions — that they would, if disaster struck, be each others’ refuge.
I think this is old, very, very old, as a system of inter-relations betweeen human beings. Sure there are thousands of different languages spoken across a continent, and many smaller subgroups within each language. But these are not barriers to sharing and relationship. We humans evolved during turbulent times, when glaciers advanced and retreated and deserts formed and retreated. Humans spread out over many ecosystems, and did so repeatedly, but also repeatedly withdrew to refuge zones when ice or drought stalked their landscapes. The passage of material goods like shell and stone, that we see reflected in the archaeological record, is more likely to represent a far more important thing than mere “trade” — it symbolized relationships that transcended language and ethnic barriers. And those relationsips could be activated for mutual help.