Are garages required to buy higher quality catalytic converters than private citizens?

No, but there are reasons professionals do what they do. In this case if they buy a cheap converter that doesn't do the job they pay for both the one that doesn't work and the one that does. If you do the same yourself you pay for the one that doesn't work and the one that does. There are a *lot* of inadequate parts out there. There is a long list of parts I only buy in OEM, having learned the hard way.

In your case, you can probably find places to buy all sorts of converters, and very few of them would pass smog - it will fail the equipment inspection. In New York your converter has to be OEM or be CARB approved, which is probably most of the difference between what you are looking at and what the mechanic will install.

Main question: No. Both you and the garage are held to the same quality standard. It's just that the beaurocrats seem to be spending more time auditing the garage and not coming around to individuals homes and looking at your one truck there. Seems logical to keep monitoring expenses down.

Last question: Yes. They were telling you the truth. A quick web search shows it's all legit. You need a certian certification of catalytic converter, and it can't be used.

They won't guarantee that the converter works good enough to keep the CEL from coming back on with a P0420 code set if you supply a cheap after market converter.

Expensive OEM exhaust parts bolt right up and seal effectively. Cheap exhaust parts do not. Then you have parts that bolt up but are mid priced. If it has to pass Emissions then OEM is your best but albeit most expensive option. P0420's can be tough to get rid of. Do the o2's at the same time.

"It fits" does not necessarily equal "it works correctly." If the guys who sold you that one will guarantee that it will pass an emissions test, you're fine. Otherwise, you're ultimately better off dealing with the garage, which has no choice but to offer the proper device.

I trust ROCKAUTO.com

They tell you if legal in NY...

See what theirs cost.

No. They are looking at getting more cash out of you. Stick to your guns and try and save yourself a few pounds, dollars, yen.......

No, they are LYING. They probably bought the exact same one you bought.