The vendor's inventories are tied to the act, difficulty and character's level. New attributes and items will be available at higher levels, acts, and difficulty levels.
There are three ways to reset a vendor's inventory:
1) All players have left town
2) If not all players have left town, then any NPC who has not been (and isn't currently) trading with any player for the previous four minutes
3) The Gamble stock is regenerated each time a player clicks the Gamble option. If another player comes to gamble, he will see the same items as you and he must leave that screen before new items show up.
Note: a Vendor's Inventory is reset if you leave town but not the act. For example, it won't be reset if you were to leave the Rogue Encampment to go the Bloody Foothills, while it would be if you step out of town into the Blood Moor.
Different vendors will not buy your items at different prices, but there are caps to what they will give you.
Act 1: 5,000 gold
Act 2: 10,000 gold
Act 3: 15,000 gold
Act 4: 20,000 gold
Nightmare: 30,000 gold.
Hell: 35,000 gold.