The store I work for will take guppies as donations (to sell, not feed), and buy (cash or credit, seller's choice) any common livebearers EXCEPT guppies "at cost"--i.e. the same price we'd pay for the fish at a wholesaler, which generally averages about .50-.70 for platies, swords, and common mollies, and a little more for fancy swords, sailfin mollies, and Endler's.
For some reason, guppies raised by the casual hobbyist seem to be more fragile and more likely to be disease vectors than any other common hobbyist-raised livebearer--they're more likely to bring disease in, and they're more likely to catch something that had been residual in a tank. We used to buy guppies, but no matter what we did they were more trouble than they were worth, so now we take hobbyist guppies as donations only. More than once, we've had the same person bring in guppies and platies at the same time, that grew up in the same tank, and go into the same store tank, and the guppies all take ill and die while the platies thrive. My personal theory has to do with the extreme inbreeding in a lot of fancy guppy lines at this point in the hobby, as to why they seem weaker than the other "fab four" livebearers, but I don't know for sure.
We keep a couple tanks unofficiallly "set aside" for hobbyist fish and don't generally mix them with the fish we buy from the wholesalers. That way, all the animals are less likely to take ill (being exposed to things that were present in a mass breeder/wholesaler tank that were not in a private tank, and the other way around), and we can truthfully say "all the animals in this tank were bred by local hobbyists", which some people prefer to buy than fish from a wholesaler.