Don't sell fish to Walmart they will be treated horribly. Since they are veil tails are so common and least desirable of all tail types you will probably end up giving most away. Chain pet stores like PetsmArt and petco won't take them because they get them from huge farms, if you got lucky and they want them the most they will likely pay is 6 cents a piece. And mom and pop stores might take them but again the most they will probably pay is a couple cents. If you had a different tail type in a good quality you could sell them for a little bit more, but 99% of breeders doing this don't do it for money, most never break even, we do it for the love of the fish, to better the species, and get show quality fish.
I suggest I'd you still want to do this to get a different tail type and look for a pair on aquabid or eBay, that way you will likely get a few from each spawn you could sell on those sites.