Unless you have a warranty through this used car dealership, I'd have to agree that recharging the A/C is up to you. A used car isn't new, and the dealer is right, refrigerant is a consumable. If you recharge the A/C and it still doesn't work properly, there may be other issues you'll need to correct. Without a warranty stating what specifically is covered, sad to say you're pretty much on your own. 
