"when you are filtering by [field] actually don't filter and display the stuff in the list, just make it impossible for the user to purchase at checkout."

... Why do we even have filters then? :/

I get some marketing guy believes making the user do the whole process helps sales, but...
1) In reality this just makes users not use your website and go on a competitor's
2) this is doable if you have 2-3 entries, not on 100+ entries, duh!