[Rachelsdad]

As I mentioned previously, PartsGeek appears to be an aggregator, meaning they don't stock any parts of their own, nor do they have relationships with any real wholesale channel. Instead, they seem to be paid on commission for selling parts maintained by many different retail vendors (each of varying quality and reputation).

Listings are sometimes (often?) truly outdated (or simply wrong). I have found various known-to-be-unobtanium parts for my 2006 STS-V (fuel filter for example, shared by the XLR) listed on PartsGeek as "in stock" only to find that the number was long ago superseded, and then the new one discontinued - also years ago. I haven't had the intestinal fortitude to try searching for parts for my '63 Bonneville or '56 Safaris.

Rock Auto, OTOH, seems to be a real parts vendor. That doesn't mean that their inventory or even ability to order every single part in their catalog is ever guaranteed, just that they seem to take retail orders and either fulfill them from their own warehouse(s) or order from wholesale suppliers who then either drop ship or deliver to Rock Auto first.

GM Parts Giant (and a slew of others with similar names) operate similarly to PartsGeek (though GM Parts Giant seems to have a better handle on what's been discontinued). I've also only seen them order from GM dealerships. Still, an aggregator is an aggregator. You might do just as well ordering through Amazon.