Crazy stuff! As a consumer you need to be really onto it!
Someone I know has a very cute online clothing store. She makes things look beautiful and trendy however, her stuff comes from those type of sites - its very common.
Fidget spinners were another example. They were in high demand and selling everywhere however if you bought lets say 10 at a time, you could get them for something like 2-3 dollars a piece. Toy world had them selling for $15 at the time.
Kids at schools were making money doing this. Same thing with the Japanese foam kawaii squishy toys.
Be clever
Shop wisely
Vanessa