sales by item/ sales by items

[idiotmike]

Dear all,

I like the ability to run sales reports by items, which enables more detail,

the "which" refers to the ability? or "sales by items"? and please explain to me why? Thank you.
and it is sales by item or sales by items?

Thank you.

[Tdol]

Itemised sales reports

[idiotmike]

what if I want to say it in terms of "by", which way is more correct? thank you