When you see something on those sites that should cost a hundred dollars and they're selling it for $12.....well, there's your sign.
My sister ordered a pair of sneakers from one of those places and what came looked like a really cheap pair of slippers. Absolutely no relation to the picture.
They also put the brand names of really good shoes like Clarks in the ad, but when you get them, there is no brand name on them. They're just Clark rip-offs. If they're real cheap but wear OK, it's not the end of the world, but if you think you're getting Clarks and you DON'T get Clarks, then it's a rip-off.
I learned the hard way a couple of years ago. I ordered a lawn ornament from one of those places off Facebook. The videos of the ornament spinning in the breeze was lovely and the price wasn't bad. After I got it and it bore very little resemblance to their ad AND I had no luck getting my money back, I saw a follow-up from the REAL artisan of the lawn ornaments and he said they were using videos of his products (which sell for 100's of dollars) and then shipping inferior stuff. Really inferior.
I learned my lesson. No more ordering anything from Facebook. Amazon's bad enough but at least you have a chance of getting your money back.