For some reason the bought food seems to work in situations where gathered food doesn't. So if you find one doesn't work and it's the right type according to animal lore, try the other.
The NPC farmers and bakers etc have some great items hidden around UO. I only learned about some when I trained my chef and thief and had a good explore to see who sold what. And peeking in containers to see what I could "borrow" hehe. Some good deco semis are out there when you know where to look.
Wenchy