I was in a LFS today and i saw a very unfortunate diano. I'd like to mention this to the owner, but I wanted to ask how it happens so I have a bit of knowledge to fall back on. Sold as mixed dianos - the very misshapen one in middle.
Although caused by genetics (which is possible, but I doubt in this case), it can be caused by poor water quality (which subsequently means vitamin deficiency, the result from an infection/disease, etc), which I'm suspecting because all the fish look thin and have reddened fills (probably not normal and from ammonia burn) and look generally in poor health. They're probably overstocked too.
What you are doing is noble, but living in the world we live I would suggest you to save your time.
That danio is severely deformed, maybe from Tuberculosis or just a birth defect, the danios all look weird really, long and skinny, nothing like the danios I have seen... Their gills also look weird, but some fish just have weird looking gills (some angels I have seen have gills similar to those and they are in pristine water), and when I googled for albino danios most of them had gills like that so I am guessing they are fine on that aspect.
The fish selling industry is complex subject, people cry about fish being kept in horrible conditions but they also cry when the prices go up (not saying this is your case).
I dont think you will get much from confronting the seller, but if you want to anyways you can just inform him that those Danios are in a poor state. You can inform him that their fish have severe deformities and that all of them look skinny and in bad conditions. You can also tell them to test their water.
But at the end of the day, they will just say: "Thanks, we will check it out" and the fish will die.
Sad, but that is what you will see in most chain stores. I breed Mollys for looks and personal enjoyment and for the care, time, food and electricity I put into those I would probably ask for 2-3 dolars (I live in Mexico, so bear that in mind, I am just using an equivalent) but if I tell them I want 2-3 bucks for each fish most people would just laugh at me and buy a Molly at Petco for 70 cents so I dont expect much from breeders and companies whose main goal is to make money.
He doesn't look deformed to me, it looks like the poor fellow is starving. And they all appear to have some inflammation to the gills. I personally wouldn't buy from a store that kept stock like this. Luckily, my LFS is very conscientious about where they get their stock. Sometimes, Petsmart gets some cool fish that interest me, but I try not to buy anything from them because everything just looks so abused. I happily pay double at my good fish store.
Spine deformities are extremely common in zebra danios, especially the glofish variety. Some live with it just fine and others don't do so well. I have a red glofish danio that is the boss of my tank and he keeps chugging on with a gnarly deformity. He came with the tank that I bought from craigslist over one year ago and was full grown when I got him. I had another that recently died and was incredibly deformed for his six month existence. I've had a few others that barely made it a month after developing the deformity.