Players will offer to buy an expensive item from you for well above market price. They will offer an amount of money ending in a 1, for example, 1461m. They'll flash the money to you in cash, but when you trade them again to offer the item, they'll offer 10x less (only 146m in this example).
It's easy to skip over the 1 and merge it visually with the capital M, so if you're making large trades, watch out. Always doublecheck the 2nd trade screen. Nobody will back out of a trade deal because you took an extra 5-10 seconds to check the trade is legitimate.
I would be curious to know if this scam, or the previous one I mentioned, could be reported somehow. I suspect not, but maybe I'm wrong.