The usual follow-up question is “But what about the Gerudo Lady in Goron City?”

Well, Ramella does pay more for gemstones, but only around 10% more than the usual selling price, which means that she buys diamonds for ♦550 per piece. You still lose ♦150 per trade this way. There is simply no way to turn this scenario into a profit.

Oh and Ramella doesn’t buy Luminous Ore, so forget about trying to sell those to her directly. Just sell your surplus to literally anyone else instead of trading with Ledo.