Breeders keep a small number of the best of the spawn to continue their lines. The rest are sold or adopted out depending on the quality of the fish. Fry with serious deformities are culled.
Some breeders sell there fry to pet stores. I would really hope that these breeders have a relationship with the pet store and know what level of care is given to the bettas while at the store. IMO there are better ways to rehome your fry.