From wikipedia:
Contrary to popular conception, there is no evidence that societies relied only on barter before using money for trade.[73] Instead, non-monetary societies operated primarily along the principles of gift economics, and in more complex economies, on debt.[74][75][76] When barter occurred, it was usually between strangers or would-be enemies.[77]
Yes, and then there’s the one who always receives gifts but never contributes. The free rider problem is present even at this tiny scale. Reputation takes care of it, generally, when the friend group decides to stop hanging out with the non-reciprocal individual.
I haven’t heard of any proposal that could scale up a reciprocal economy like this up to a city of thousands (let alone millions). The issue is Dunbar’s number: our brains simply cannot track relationships with thousands of people.