When I go to "start a swap", what are the users sorted by? Obviously, they do not follow the pseudo although it indicates so. When I refresh they become completely different. What is it?
The users are sorted randomly. But as you can see on that page (Click on a column header to change the list order) you can easily change that order.
Interesting headers are "distance" (click twice because the order starts with the highest distance first) to swap close to home to avoid high shipment costs, or "Coins that may interest me* which gives an order with the highest amount of coins you don't have yet at the top.