I have some KOs of old transformers which you can't buy (mostly are classes as the 'molds are lost or too damaged to be useable or worth remaking). Some brought in cheap UK shops. When they are clearly marked and priced as KO, then I have less problem with them being such. Like some third party toys. If they are honest and priced honest, it's a bit less bothered but.. some bits are a bit.. iffy.
It's kinda like, I've done some 3D model design. I wouldn't say stuff tooo fancy but nice and people liked them. But they were free. Because they were reproduction aparts. Not perfect, but If I was missing a part of an old toy which the prices for some of these parts are.. insane, then I took some photos and recreate it well enough so the figure looks good (a good example is Transformers: Head Masters. if you lose the head, which is removeable, you are stuck with a robot with no head. This in fact lead to a 2008-ish toy being made where there is a small spring loaded head so if you lost the main transforming head, you atleast had A head). one thing that annoyed the hell out of me was people that did that, or just 3D scanned the real ones and THEN SOLD the STLS. For me, that just crossed the line with some of these bits. You can get away with selling prints as 'price of printing, materials etc, but my view was I didn't create anything. I tried to recreate something which was designed by someone else. Sure it took a fair bit of work to do, but still.
without getting into some legal bits, It's like If you have one shield of a certain design, and you need a bunch of them, so you make a quick and dirty mould to make your own copies of them. I think that is one thing, but to do that and then open up a eBay or more likely Etsy store selling your shield copies... ugh.. Though not against making up some for a friend, who made cover time it has taken (not that it'll be much).
But yeah, I think the huge problem are ones who openly bootleg and try to claim they are innocent nice people just helping people with these models which are out of print and no-one can buy so they are clearly not in copyright etc. Did you know that was ONE (there were a few) reasons that the 60s Batman show wasn't on DVD for a long time? eBay had solds of copies where the guy claimed 'it's public domain cause you can't buy it' and eBay would do nothing cause he claimed it was public domain even though it was clear it wasn't. But things can get very messy.