One of the best things to do when buying via the internet is to ask for feeding records from breeders. Most all of the established breeders keep meticulous records with their snakes so everything can be tracked when questions and issues such as this arise.
Now, one thing to keep in kind is that these snakes sound like they were shipped to you, so they may still very well be going through an adjustment phase. The weight loss could be a concern and as such, a trip to your vet may be in order just to make sure it's in good health.
Unless you see where others have left feedback about the vendor you traded with, I wouldn't jump to the conclusion that he tried to pawn off his headaches onto you. I would suggest staying in contact with him and between the two of you, hopefully an amicable solution can result. I wouldn't write these pets off just yet though. Some take longer to adjust than others. Patience and care are the keys to helping them adjust to their new home.