The unfortunate answer is that replacement is likely cost prohibitive. Shipping costs are never on the vendor, and that leaves you as the one who has to eat it. Since you split the shipping cost on a group buy already, chances are you are in it for a lot less than you could be. There is a bitter pill to swallow when you realize you have to pay a 3rd party a wad of money just to deliver an item that should have been right in the first place.
As far as the vendor goes, it's hard to say. What they *should* do is refund a portion of the money, but what they will likely do is provide some sort of credit. I think history has shown that gill curl is an affect of poor water quality over time (at least with arowanas). I guess you can assume that is due to the collection facility, but at this point I'm only speculating.
On the other hand, it is repairable. I've seen pics of hardened gill covers trimmed down, and they grow back after a while. So it's not like a dead fish was shipped to you. There is some value to the fish, but unfortunately, it's likely always going to be something that you're less than happy about.