Hi,
I can't say that I withhold the truth of the matter, but as far as I've seen, the same towns are always displayed. They might be more or less depending of your trade skill. The ones that show the prices aren't necessarily the best trade deal, but they tend to be close to where you are sending your caravan from.
You can always just asses the prices in the market and then talk to the mayor.
For further information check the mount and blade wiki for /Merchant_Caravan
All the best