The main problem they have, I imagine, is sparse data. There are only certain fields (depending on category) which they force sellers to populate, eg: "name", "brand", etc. Item weight (which is distinct from _package_ weight), and "number of units" do not seem required, and so not many items have that information filled.
So, with sparse data, they have three choices:
1) Allow filter/sort by "unit price" and do not show the X% of listings that are missing this data -- many of which the user may actually be aware of and/or interested in.
2) Don't allow the option at all, and just rely on the fact your customer will do comparisons manually.
3) Try to derive the number of units from text cues in the product name, features, and description, then do #1.
So, with sparse data, they have three choices:
1) Allow filter/sort by "unit price" and do not show the X% of listings that are missing this data -- many of which the user may actually be aware of and/or interested in.
2) Don't allow the option at all, and just rely on the fact your customer will do comparisons manually.
3) Try to derive the number of units from text cues in the product name, features, and description, then do #1.
It seems they chose #2. I'm going for #3.