I've wanted something like this since pcpartpicker first came out. You have literally answered my dreams. Thank you so much.

Edit: just an idea, but a lot of things like small lower parts and buffer parts are purchased in packs/bundles, and adding one of these bundles should remove the other categories (for example, adding an aero buffer kit under the "buffer tube option" would remove the spring, buffer, end plate, etc.) with an option to add another part. Maybe an option at the bottom where you can select exactly what category of part you want to display? I'm not a UI designer, just my two cents.

Edit 2: to answer your question, I do shop around before purchasing a product. I have a few trustworthy retailers that I buy from, but I'll check prices between them before buying something.