Dragons you have already purchased can be purchased again. Fire dragons can be purchased for 150 coins, hatch in 30 seconds, and can be sold immediately for 100 coins. If you place the dragon in a habitat and pet it 3 times, you can grab an additional 60 coins, for a net gain of 10 coins on each fire dragon. Note that you will need to purchase each single-element dragon for breeding, so you should only go to the effort of farming fire dragons once you have purchased all, or most, of your single-element dragons (i.e. fire dragon, wind dragon, earth dragon, etc.)