Sometimes the vendor will not want to trade with you due to a faction conflict. You can open the faction window and check your status with the following command:
/open faction
To check if a mob has faction, simply target it and press 't'. If you have a negative faction standing, the npc may refuse to help you.
Some older characters may not have been properly flagged for earning a garden or sanctuary key. If you have the required key and a positive faction standing send us an in-game petition. A GM should be able to assist you.
It won't be long before you can start shopping again!