It's not a good idea to move rays after feeding for a few reasons
The should be starved for 2 days before moving
If you want to see the ray feeding before buying get the owner to feed then leave a deposit and return a few days or the week after and ask the owner not to feed 2 days before pick up
It's not good for the rays stomach to move when full
It's also very bad for the water you are moving the ray in as they do crap and can be sick in the water and rays can't deal with ammonia or nitre very well
Female seam to suffer from being moved on a full stomach more than males I don't know why
I hope this helps and sorry for the late reply
If you are having any problems getting new rays feeding turn the temp up to 90 lots of air
Depending on the size of the ray feed what the shop was feeding or try live blood worm or river/ghost shrimp
Also check your water parameters as adding a new ray can have a big impact on you filteration