Your right people are focusing on the fact that you didn't quarantine your snakes and that could be in part because everyone here believes quarantine to be extremely important because it is and it was stupid on your part not to quarantine them.
You knowingly kept the snakes after you noticed problems when they arrived. You could have sent them back then and you wouldn't be in this mess. If you want more support than post vet reports and pictures, but still expect some backlash.
I agree that animals that are sick should not be sold but have you considered the fact that maybe he didn't know they were sick, I mean it did take you 2 weeks after receiving them to notice. The guy is paying your vet bills, which I honestly don't think he has to. If you want support start offering proof.
Also it wasn't the smartest idea in the world to agree to a trade/sale without seeing what you were getting.