The english in here oO
The GM in that case probably destroyed the pet because A: person was stupid enough to contact a GM saying he potentially had a duped pet and/or B: its illegal to pay with reallife currency for ingame stuff.
So if you want to pay for a legal pet/mount you'll have to pay for the expensive ones, but those are forced down in price by the honest sellers to maintain competitiveness. Being humans as we all are, we also tend to go with the cheapest solutions when presented with multiple offers. Why pay 800k for a Soul-Trader when theres a guy next to you in SW/OG thats willing to give it to you for 100k?
Just to point out, a player that is reselling a duped pet might just price it high so players think it is a legit pet.
This does not solve the problem. I am pretty sure the reason why it is possible to dupe and sell pets with no or little repercussion is that pets current has no unique ID. If this is really the case a duped item is as good as a legit item, the only thing holding a player back is integrity issues upon finding out that the item is dupe.
I think the reason why the item was deleted was because of communication problems with the GM (GM cannot fully comprehend the individual and probably delete the item due to real currency purchase on a virtual item), as Lolfixheal has pointed out. Buying a TCG card with real currency and redeeming its code in booty bay is completely legit, I am not so sure about paying real currency for a existing virtual pet as it is against blizzard T&C to sell/buy virtual items for real currency.
Look at blizzard they are good with dupe and experience with dupe items, for example Diablo 2, they just patched in Unique ID for each item and solved the problem. Blizzard also implemented a warden program to periodically check for duped items and delete upon detection. This "Technology" has been used by blizzard for close to 10 years.
I guess the reason why pets have no unqiue ID is probably due to the nature of the pet system. They will have to code it in a way that obtaining a pet and using it on an account binds the pet unqiue item information. When it is caged the item will have the same Id instead of regenerating a new ID upon caging. If I can think of it, I am sure blizzard has thought of it, but they are slow on it probably due to patching issues or discovered new problems generated upon implementing it on closed beta.