Those aren't past breeders? Or individuals that were kept in hopes they would be good enough to breed? I doubt they are keeping and growing them to such large size simply to sell them at a larger size, it is not cost effective.
These were given to me as a take them or I am going to flush them deal. So I will due every thing that I can to get them the home that they need.
I don't think a few months jumps a 'marginally priced' 3.5" goldfish to a 6" multi-hundred dollar goldfish. I think a lot of these, not necessarily all, are sold after they are not chosen as breeders or show individuals but are still of the highest quality (the breeder or shower just has even better that they want to focus on or make room for others).