One of my favourite Harry Potter loopholes doesn't even involve magic. It involves exploiting the Muggle economy. Hagrid tells us there are 17 Sickles to a Galleon. Sickles are silver, Galleons are gold. Simple enough. Except gold and silver don't have a fixed exchange rate in the Muggle world. Their market value fluctuates constantly. Before 1900, the gold-to-silver ratio averaged around 16:1, which is pretty close to the wizarding world's 17:1 rate. Throughout much of the twentieth century, however, it averaged around 45:1. See where this is going? Wait until the market heavily favours gold. Melt your Galleons down and sell the gold in the Muggle world. Use the money to buy as much silver as possible, then take that silver to Gringotts and have it converted into Sickles. Finally, exchange those Sickles for Galleons at the fixed 17:1 rate. Congratulations. You've potentially turned a small pile of wizarding gold into a much larger pile of wizarding gold, simply by exploiting the difference between Muggle market prices and Gringotts' fixed exchange rate. No Dark Arts. No Horcruxes. No elaborate schemes. Just precious metals, a fixed exchange rate and a wizard who understands arbitrage. The real question is why nobody in the wizarding world seems to have built an entire financial empire around this.