With pet store bought rats, they may not have been exposed to any treat foods before, and with their natural caution with new things, it may take a little while for them to warm up to all the wonderful healthy foods you have to offer.
That said, mine (including the pet store babies) warmed up to bananas and cooked brussels sprouts and peas *really* fast. Peas or cereal would probably be the best size for trust training, and a bag of frozen peas or puffed rice/cheerios style cereal would be super cheap. Good luck!