A problem is that this is an idiom within an idiom.
A Google search turns up:
to have a keen eye for sth synonym | English synonyms ...
dictionary.reverso.
and, to give some examples from the internet,
... have a keen eye for pigments and dyes
... have a keen eye for "fabricated" essays
... have a keen eye for details.
but I can't seem to access the promised dictionary.reverso entry.
So 'have a keen eye for' accepts count nouns. 'Have a keen eye for a bargain' is an idiom within an idiom and apparently count.
Have a keen eye for detail is a longer set expression, and obviously uses the non-count version of 'detail'. However, this does nor indicate that 'have a keen eye for details' is in any way wrong.