everything has to be compared. i could sell a car for 1XMR but it would not be a good deal. if i were the only car seller i would be forced to use usd to set the price. The second person may be able to find their car sales price compared to mine if they are close but otherwise they would need to use usd to figure it out too. The 3rd car seller may have better luck not using usd because they have multiple cars with which to compare.