What is the ruling on buying from or selling to people who have suddenly become excessively wealthy, purchasing goods (such as cars or livestock) for many times their value and paying later, then selling them at very low prices, especially given that the source of their money is unknown?

The basic principle is that what a Muslim possesses is his property, and this apparent ownership is not relinquished except with certainty or strong evidence. Ibn Taymiyyah said: The basic principle regarding what a Muslim possesses is that it is his property if he claims so, and there is no doubt in dealing with a trustworthy Muslim. The method of buying goods in installments at a high price and selling them immediately at a low price does not prevent dealing with them in buying and selling.