Well, no they dont really sell. If you can create a High Quality Strain of guppies, that breed true, and are GORGEOUS, then they will sell, for alot, assuming you sell them on like aquabid.com . But, assuming you cycled your 10g tank, you could invest in something like A Apisotgramma ( Cockatoos are very pretty) they sell, VERY VERY FAST, but are VERY hard to breed ( in my experince), and you grow them out, they will sell, the market for Cockatoo Apistogramma is very HOT, they are in high demand, and they need a god grow out tank. Atleast 20 gallons, and sell after about 4-6 months of being grown in a indiviual tank.