As said above, no economies are perfectly efficient, and traders make their money improving that efficiency. This is not a new revelation: we have over a millennium of history of people hating traders, moneylenders, etc. for "getting rich doing nothing" while they provide valuable efficiency improvements.

It gets especiallly blatant when a trader buys and sells the exact same good to generate a money advantage from its price fluctations.

Price fluctuations indicate real problems to be solved. That is what traders and middlemen do: resolve those problems. Prices fluctuate because the need for things varies over time and space and between individuals. "Buy low, sell high" means "buy something when there is so much that people want to get rid of it and sell when there is so little that people want more." The profit in between is (1) recognizing that difference over time and (2) being willing to buy something you don't want, hold it when you could be using that money for something else, and then sell it to people who want it more. If there were not value in that, you would just hold your surplus until you needed it. You can capture all the benefits that traders provide by just never trading (note: this is a bad idea).

The same applies with trading goods over space (excess in one area, dearth in another) or between individuals (finding the people who want to buy what you want to sell). The economy is not perfectly efficient, so the trader makes money by smoothing those inefficiencies and incurring the transaction costs of moving things across time and space, along with the risk of ''being wrong'' and losing the investment.