Wait 42-48 hours OR save, quit to desktop and restart the game. When you log back in the merchant will be fully restocked. =)

How often do Skyrim merchants get more money?

Merchants will generally have 3-40 gold more than listed, because their personal pocket change is added to the merchant-specific gold. Gold is reset each time the merchant's inventory is reset or every 48 hours.

How do I reset a gold vendor in Skyrim?

After selling to a vendor and all their money is gone, save your game then exit to the main menu. Open the game and the vendor's money has reset!