I emailed the seller explaining what happened in the basement and sent him a few pictures. He just called and said that they would replace the carpet and send over their plumber.
I was so surprised by how professional and courteous he was. I expected him to be defensive, for him to say that it wasn't his fault, that it is too bad because it was discovered after the sale of the house was finalized, etc. But no - there was none of that. He is even sending someone over ASAP to rip up and take out all the wet areas of the rug and padding so it does not grow mold.
Sometimes I am surprised when people do good things.
Now I can start doing more important things, like planning for my projects!