Hmmm.....I'm trying to think the last time I saw a guppy in the ocean.....
Its a pity, really, to feed them those, IMO. And I can't imagine how many guppies you would need to feed some of these eels that are several feet long.
Morays are very much scent predators; they should be pretty easily trained to go after other foods, especially things like silversides. I've worked with a fair number of eels and never were they fed live foods. It is one thing to have a finicky eater, it is another to not want to make an effort to switch them onto appropriate foods. That is what those LFS sound like they are doing, IMO at least.
What else have you tried feeding the eel? Tried any silversides, smelt, etc?