My lfs offers a dollar of credit per guppy. You can also use them to fill a really big tank or indoor pond. You can give them away freecycle or Craigslist, but at some point you will run out of demand fire guppies. That will probably take a long time
I often get guppy baby's end up with hundreds of them I just keep the fry in a big breeding box without the parent until they get big enough not to get eaten or to sell them or give them to someone end up breeding like crazy though