A warband can buy any of the items on their starting equipment list at the cost listed upon creation. But after your first game you must abide by the rarity of the items and the cost associated with those items listed in the trading post.
So for starters, you can equip all your guys with dwarven axes (You can even buy extras). But after game 1, they become rare 8.
The other thing to note about this is that Gromril armor says it costs 75 gc on the dwarf equipment list. If you buy gromril armor when you create your warband you pay 75, but after game 1 you must search for gromril armor as it will be rare, and you will be paying 150 gc, not 75. Even though it specifically states this in the dwarf warband list, it applies to all items. So a human mercenary warband still has to pay 30gc for a dueling pistol and has to roll for rarity after game 1, even though it is listed as 25gc in the roster.