"She sells more of chicken than of fish" was listed as the right translation here. But it sounds really unnatural to me (a native English speaker), a bit like broken English. We just wouldn't put the "of" in there.
I said "she sells more of the chicken than the fish" which was wrong, but I'm ok with that as there is no "la" in the phrase, fair enough. What I wondered is if "she sells more chicken than fish" is accepted because that should be the correct one!
Also, being that "fiŝaĝo" means "fish (meat)", it's more than likely that it isn't a fish she is outselling. Unless it was, "She sells more chicken than Fish-meat," where "Fish-meat" is a person, a name of a fish or literally a big old pile of fish meat selling chicken O.o, then in this case it may truly be ambigous and would come down to context as to which one you meant.
I'm not really sure myself, but as "da" would indicate "a quantity of", the meaning would clearly be "more chicken than fish". Maybe this would be an example of de vs. da? Also, it could be that "pli da wouldn't be used for "more than a fish". I would love someone with a bit more knowledge on this to weigh in!