I agree, I generally stay away from wild caught rays unless its from a source I 100% trust. I've had my fair share of wild caught rays that eat nothing but live fish and black worms.I find all this a little hard to stomach! It almost seems like selling rays in this way is acceptable, surely there has to be more to this than making a living from these amazing fish. Even if the ray is feeding which in most cases they probably aren't! goldfish in 90% of cases carry external parasites such as Trichodina, Gyrodactylus in some capacity, intestinal tapeworms are also common. Getting new imports to accept frozen/ dry foods before offering them for sale is a tedious job at best, you can see why they are sold in this manner, no work involved...easy money basically.
It beggars belief that anyone would buy rays that are sold in this way, these are highly intelligent fish that require a certain level of skill and experience from the aquarist to give them what they need to ensure they live as happily a life as possible, a life that can easily exceed 25 years.
I feel sorry for the fish here "not" the buyer, if anyone is silly enough to purchase any ray under these circumstances then they must be prepared to suffer the consequences .
Prazi pro is a good start. Wild caught rays are usually teeming with parasites and if they come in fine, they'll probably get something if they get fed goldfish.
Good luck!