You got mad that they couldn't put a bulb in? It's not their job to do that, they just sell parts. A mechanic installs parts, so getting mad that they couldn't do a job that everyone in the world CHARGES for, and were willing to try to do FOR FREE seems pretty stupid.
They can only test certain cars, mostly newer, and you never paid for anything so you can't "Not shop there again" when you weren't a customer in the first place.
You also didn't pay anything for the service. If you want a trained professional working on your car, take it to a mechanic. (Who will charge you, but hey that's the breaks)
It's his job to sell you auto parts, it's YOUR job to know what you need and ask for them. If you want all the parts guaranteed right and all replaced, take it to a mechanic who gets paid to know your engine inside and out. Retail auto parts store are NOT mechanics, they're there to sell you parts, nothing more. Also, how hard is it to measure a belt? Get a tape measure and measure it.
