Fraudulent parts on Amazon is a mix clever and just plain dumb I got bricks instead of a GPU. For work I’ve gotten a handful of hard to notice frauds. We got some Samsung SSDs the older sata drives not NVMe. One was a different shade of black and looked altered. It had the guts of a USB drive with a usb to sata homebrew conversion. That one Amazon took back no questions but it was a company purchase so they probably treat those differently.

For Legos if they were frauds I would imagine it would be noticeable. It’s probably a discount because of the volume they purchase.

If you buy legos often I’ve had some good luck going to goodwill stores and getting them super cheap. It’s not going to be a set. Usually it’s mixed pieces in a vacuum sealed bag or bin if there are a lot.