Always wondered if these are part of money laundering operations.
Expensive watches and art definitely are.
You give someone $50,000 Swiss watch. When they are in Geneva, they resell the watch for a good price for a reputable Swiss merchant, take the money and walk across the street to the Swiss bank. The same shop can resell the same watch again and again.
Untraceable transactions don't solve the money laundering problem. That's why drug lords end with warehouses full of $100 bills.