It's fairly common to cull, freeze, and sell kinked/misfit snakes as feeders for other snake-eating snakes. I never heard of selling them live at expos.
I would never purchase a snake with RI as a feeder, too much danger of cross-contamination and a lot of collection-killing viral diseases initially present as a simple RI. If their owner isn't willing to treat then he should either cull the snakes or give them away to people who will take them to a vet.
As to where to report:
https://ball-pythons.net/forums/foru...iries-Feedback
There are some Facebook groups where you can report the seller, including:
FBI - Feedback and Inquiry
Herpetoculture Feedback and inquiry
Herp Industry FBI: The Good, The Bad, The Truth
Finally FaunaClassifieds no longer has their Board of Inquiry for posting feedback, but if the vendor you saw selling sick snakes posts his own for-sale ads on that site you can post feedback on the seller just under his ads as responses; you can not create a new thread for feedback.