In the specific sector of car parts, search is based on the year, manufacturer, model, and trim line/options of the vehicle. What's weird to me is that every one of these retailers has a point of sale system that works on the "specify vehicle, list and search within parts" model, yet they're using a traditional/"Amazon style" search on their own website.
RockAuto's site is ugly, but it's extremely functional. Their business model has complicated shipping (sometimes buying a more expensive item from the same warehouse as another item in your cart is less expensive after shipping than buying a less expensive item from a different warehouse) but there are UI affordances that make this easier to deal with. It's kind of sad that "show the shipping cost in the shopping cart, on the same page as the product list" is a noteworthy UI affordance, but it's so rare. (Also the option to select several items that meet spec, and it will automatically select the overall lowest cost after shipping set of products)
I went to their site to see how ugly it was, and was absolutely blown away by how visually terrible it is. Truly function before form.
If they could just include shipping when doing a "sort by price", or a price limit search, that alone would help SO MUCH.
I pick "prime only", "under $10", and I get results where some other non-prime seller is selling it for under $10, with $15 of shipping. Useless.
Actually even ignoring shipping, if the under $10 price is for a non-prime option, why is that being included in a "prime only" search for under $10?
Honestly, Amazon's own product search is pretty terrible too, unless you're searching for a specific name-brand product. Google might not even have to be very good to be competitive.
The best product search I've actually found is NewEgg's (for computer parts, specifically). They're the only one who's collected enough accurate product metadata to facet their results effectively.