If it's advertised as a case and it works, why is it a scam? I don't own a 3D printer, nor do I know anyone who does. If someone is designing, printing, and selling a useful product, why is that a scam?
Because the sellers are categorizing them as computers on ebay and filling in all of the appropriate specifications as if it was the computer, and only putting in the description "CASE ONLY, DOES NOT INCLUDE ANY COMPUTER PARTS".
Automatically no 3d printed case is worth $300 because it's not possible to make it good enough quality.
A metal and wood case could be worth $300.
A good design is worth something, and a good printing job is worth something, and the convenience of a service is worth something, but no amount of good design and quality printing job and service convenience can make a 3d printed case worth more than about $50 before shipping, and it would need to be somehow pretty special design to overcome some basic material and process weaknesses and still be functional and look good to be worth that much.
I think they are saying people mislead customers into thinking they are getting a "$60 gaming pc" when they are actually just getting a 3d printed case with nothing inside.