I wouldn't say you aren't leaving them to die at all. It's up to the fish store to feed them or medicate them to get them healthy for sale. Skinny=unhealthy (or they are starving them) Unhealthy rays will not take well to the move from the store to your home etc. it could kill them in a hurry. You want a healthy ray to start with so that the stress the ray experiences in the move does not turn out to be the straw the broke the camels back. Let the LFS worry about the sick ones.
I would also make sure the ray eats in front of you at the store. Rays generally will not refuse a meal. (I think you said you did that) Another suggestion... make sure they haven't been feeding the ray live feeders at the store if you have any intention of keeping other fish in there with him. I successfully keep small tetras, sword tails and a couple gourami with my humerosa. He was brought up on earthworms and krill and sometimes I'll treat him to ghost shrimp (live). He NEVER bothers any of the other fish (yet anyway). It's like he doesn't know the fish could be his dinner!
Anyway, good luck.
