In reference to any single item offered for purchase, should I say 'sale item' or 'sales item' ? 

Experience tells me it should be 'sale item', because 'sales' seems to be preferred in English for things involving, at least potentially, an arbitrary number of individual sales, which is not usually the case with a single item.

EDIT 1: 
And if it's not a single item I'm referring to, but rather a 'model' of individual items, thus an abstract collection of potential things, is it correct to say 'sales item' then? For example, as in 'The shops's sales items encompass the freshly released processor (model) CPX'.

EDIT 2:
This question is not about the ambiguity of 'sale' or 'on sale', or the on-sale–for-sale dilemma. I ask whether it's OK to say 'sales item' for a particular item being up for sale (regardless of context), and, more importantly, if it can be reduced to 'sale item'. Why reduce? Because it's economical, and, above all, one and the same item is only sold once, usually. Whereas, for example, the term 'sales tax' covers an unspecified number of instances of selling ('sales').