Epic

If you only have one epic trinket or ring that meets the criteria for this achievment, you can ''exploit'' it by simply equipping it in the other trinket/ring slot, and it will count for both.

To elaborate on what quegabriel noted so correctly:

Patch 3.1 says:

User Interface

• Players can now return items purchased with an alternate currency back to the original vendor within 2 hours of the purchase time for the original cost of the item. Stackable items (such as Frozen Orbs and gems) and charged items that can be purchased with an alternate currency are not eligible.

This means:

1. Get some Honor/Conquest/Justice/Valor Points - whatever will get you an item that is itemlevel >= 213 for a slot you don't have the achievement part for.
2. 
   
3. Go to the appropriate vendor and buy the item.
4. 
   
5. Equip it.
6. 
   
7. Get the achievement part.
8. 
   
9. Sell the item again, getting all of your points back.
10. 
    
11. Repeat.
12. 
    
13. ?
14. 
    
15. Profit!

E: Updated for Cataclysm. :)

I think they should change this achievement to "Pimpin in Purples" ...just an idea.