I've actually had really good luck so far with ebay. Parts are usually anywhere from somewhat to insanely less expensive, and I've not had an issue yet with fit or compatibility. So far I've obtained complete brakes, muffler/exhaust pipe, wheel bearings, and a few other odds and ends from eBay with no problems, although I always go to RealOEM first and get the part number, then paste it into the eBay search bar. Nearly every other on-line parts source I've checked out is at least as expensive, if not insanely more so.
An excellent example is a new Idle Air Control Valve. I was able to clean mine, thank God, so I didn't have to replace it (yet). However, when I was looking for new replacements, I found that Pelican wants well over $330 for a new one. I've seen some as high as $450! On eBay, I can get a brand new one for around half that. $170 still isn't cheap, and I'm sure it probably isn't quite the same quality as OEM, but, it's a lot less than Pelican or anywhere else I've looked! So far I've only encountered a few things that are just as expensive on eBay as at the parts stores, or other on-line sources.
What I can't get on eBay I usually can get from the dealership for about the same price as anywhere else.