In case the group needs some ideas to move the plot along (since I'm assuming your characters know the campaign world better than the players do), here's some ideas you might consider as far as moving things along:
If you have any method to teleport the group, you could pick a city (probably one of the virtues) and teleport there. Once there you could request an audience with someone in charge there and explain your situation. This is probably the simplest option to move the game along.
If you don't have any fast travel methods available today, or you want to wait till tomorrow to prepare them, you can camp. You can either wait at the site of the battle, or try to get out of there. The bulettes are a travel option, although you might have reasons not to use them. The desert is dangerous, so there's always the chance of more encounters, but that could be a lot of fun as well.
You could just set off to travel, or send a message using some kind of spell as well. This option is perfectly valid, and gives time to somewhat explore the desert (which would in turn let me introduce more about it). It would also set a decent pacing mechanic where we could keep things active for a while.
Or you could try seeking out the nomads or the remnants of the merchants in the desert to find out what was going on. How much did either side know about the attack? Did anyone expect it? What is the history of the items you recovered?
This list is of course not exhaustive. I just figured that giving some ideas might help (since I'm trying to get things moving along). As a play by post, I know leaving everyone "high and dry" after the first fight can slow things down a lot, but for story reasons I want the players to decide how to approach things. It's entirely up to the characters how they want to react. You could leave the treasure to the desert and just leave if you wanted, I'm leaving the options open to your discretion. Don't feel the need to follow my lead if you don't want to, thats part of how I want to run this game.