Odds are, those kos are from random gutter-trash families that got picked up by Mister Pest Control. Selling them on the cheap is more economical than disposing of them in a non-sales-y fashion, and creates a small but significant chance of the kos getting away from humans, breeding, and giving the pest control guys a way to justify their jobs.
The only thing is that there are a bunch of rare types in there. 3$ would get you all of those, and you could probably get more than that from a petstore for the Satori and Sikieiki.
The way I've thought about it is that although the number of images in which the 'rare' yukkuris appear in are way smaller, in-universe they wouldn't really be all that rare. They reproduce in the same ways, their lifespans are mostly the same, and yukkuris generally reproduce a hell of a lot...
Personally I think that these would be bred yukkuris... for the gutter trash, I think a more efficient, effective and economical use/justification for them would be to send them to special yukkuri facilities and use them to safely and cleanly dispose of garbage.