This seller clearly knew the birds were sick, hence refusing to let you see the very obvious signs of illness before you paid for them. This seller knew they were diseased and sold them as healthy animals. Even had the nerve to intimate that YOU would contaminate their CLEAN environment...
That said they should take the birds back and refund you the costs.
But you cannot hold them accountable for your birds' illness since you chose to bypass all normal quarantine procedure, and especially with clearly ill birds. If you've been paying to treat their birds, instead of taking them strait back, that was your choice as well. You can either keep the birds, heal them up, and move on. Or you can try to return them and get a refund of your purchase price, though not necessarily any costs of treating them. Really depends on just how scrupulous (or not) the seller is.
Either way you go I'd definitely do what you could to spread the word that anyone in your area (anywhere really) should be sure to check each chick, beak to bum, before accepting delivery. Good luck. Hopefully your birds will get well quickly.