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Is legal sending dollars to someone in Mexico, and sending them back for profit?
It is certainly legal to transfer money between people, no matter how often, as long as the money is not originally from illegal sources. If you are gaining in the process, you need to pay taxes on your (net) gain, as on any income; but as always, taxed income is still income. Consider the accumulating transaction cost, the inherent risk (of your friend keeping the money), and the risk of the exchange rate going the other way; but otherwise it is a simple arbitrage business. There are thousands of people who do that all year long at stock exchanges and money markets; you might be able to do it more efficient there, and you don't need a 'friend' on the other side for that.