I think if that damage were there before your tire blew, you would have felt it, so I wouldn't put it on the seller yet. An old rim with old spokes can sometimes spontaneously give way, especially if things were not quite set, like with a loose spoke (could have been the case since so many threads were showing on that spoke). It's definitely bad luck, and that sucks, but I suspect that might be all it is.

The rim should be replaced, as others are saying. On the upside, the derailleur hanger can be bent back easily with the right tool, and the derailleur, at least from that pic, looks like it may be just fine.

I feel for you... I know how great it feels to get out on a bike you're excited about.