I don't know if I should jump in here... but I can see some validity in both y'alls sentiments.
I tend to use larger suppliers and some local stores for parts, as well as buying off eBay (mostly NOS and good used). I have done Rock Auto and mebbe Amazon once for something. I've ended up with some stinkers even from the local parts houses... we've gotten to an era where much of everything has been off-shored to god knows where, and it ends up in all the parts channels. One example is mechanical fuel pumps, which I've heard enough bad about that I resorted to locating older parts stock. Many rubber parts coming out of China or Bangkok or wherever are garbage, and the Chinese can't make a sealed beam headlight to save their ass. Most of the bargain price catalytic converters are garbage; they saved money by scrimping on the precious metals in the catalyst, and the things usually fail in a year or so. I need a wiper/cruise switch, and noticed that the price has gone from about $55 15 years ago to $16 today... I think I'll be looking for one that's not made of Chinesium.
At the end of the day, an oil pan just needs to hold oil, and as long as the holes line up, it'll probably do the job... Let's hope they didn't crossthread a drain plug made out of cream cheese into the thing.
It seems like the online parts channels are a mix of decent quality stuff, kind of okay stuff, and some sh¡t that's totally unusable... but they occasionally have actual OEM parts. One other thing that comes to mind, and it affects both conventional parts houses and online suppliers- is when some podunk company buys the rights to use OE names on their products... The parts guy is rattling off prices and warranties, and says: "You can get the Bosch for $78.99". Ehh... sure. Bosch never made a Delco CS130 alternator... it's some sweatshop, err... rebuilder that bought the rights to use the name. Just do your homework, use your brain, and you probably won't get stung.