I'm pretty sure I got factory seconds buying some jeans on Amazon recently. I got a pair where the waist size on the inside tag was different than the one on the outside tag, and another pair of relaxed fit jeans with a regular fit tag on them. I think they were authentic, they were just factory seconds that somebody bought to sell on Amazon.

You simply cannot buy clothes online. It's far too unreliable in every way. Even if they deliver what the advertise it's hard enough determining what size to buy when in person, the amount of work it takes to buy something int he right "size" online is astronomical. Easiest examples are shoes and pants. I can wear size 8.5 to size 11 shoes or 29 to 33 pants. These all fit the same.

I've found the exception to this is first-party webshops. For example, buying Patagonia gear from Patagonia.com is by far the easiest way. Their sizing is consistent across their line, and the website has a much better selection of colors and stock than any reseller does.

Yeah, that seems reasonable if you're unable to walk into a store. Even if the sizing is internally consistent it still seems difficult for a first time purchase though.