You screwed up big time, buddy. If you had done even minimal research on bearded dragons, you would
know that putting them together, even if they are the same size, is very dangerous. I have a rescue that is missing both left legs from another dragon biting them off when they were both babies.
Your mistake cost an animal its life, and you need to man up and accept that responsibility, not try to pass it off on the seller. You screwed up.
You're also housing juvenile dragons together, even though I'm sure by this point somebody has told you not to do that. If you can't afford separate cages with separate UVB and heating bulbs, you can't afford to keep dragons, period.
I would
never do business with you, based not only on your immature behavior here, but your complete neglect of the animals in your possession. I only hope that your remaining animals don't suffer further injuries because you insist on housing them together.