That was one of the things I enjoyed about guppies. If you had too many excess, you could always sell them off as food for other fish or snakes(I think)...
I wouldn't try selling them as bait for wild fish, no need being the cause for a fancy guppy invasion. Not that they would probably do to well, but better safe than sorry.
Also, if you have chickens, they LOVE guppy sized fish. I used to catch minnows and feed them to my chickens, and I tamed even my most skittish bird( a gamehen from out in the woods) with those little fish.
If there are private pet stores around, you can usually sell fish to them. I know that a local pet shop would buy anybodies extra, as long as it was a reasonable amount of fish. Not just two extra guppies, but they would buy 15 or more guppies.
Just like with any other animal. Those purchased in a store are "hatchery" quality. If you research certain types of fish online, you will find what we consider "show" quality or "breeder" quality. It is really funny actually, to be able to purchase a guppy at a petstore for $3-5 a piece and then go online and find them being sold for $50-150 for a trio of some "special" strain of guppies. Or to go to
Walmart and purchase a Betta for $5-6 for one and then go online and find them for $10-40 a piece!! It's crazy.
If you are going to breed and raise fish for sale. Find a type you like, do your research, buy good stock(not petstore stock), and expand your market.
-Kim