You can have every Charizard mega-evolve. The quest is for the megastone, which can be used by every Charizard.
You just have to finish the quest with that Charmander you showed to Oak and you will be able to mega-evolve any Charizard you want.
+1, but I feel like this should only apply for untradable cosmetics from bosses. Getting multiple of the same untradable cosmetic as a rare reward from a drop doesnt feel good.
c.o. = current offer. Since youre saying the auction will start when you get the first offer, I guess you mean s.o. = start offer?
Whats the duration of these auctions?