If they really cared about users not getting frauded they could just make the Raspi firmware show an ugly bright unskipable warning when detecting a mismatched RAM chip but still allow it to fully use the new RAM after acknowledgement.

Besides, and please correct me if I'm wrong, if fraudsters are already bothering to un-solder and resolder the ram chips, couldn't they also flash a patched EEPROM to bypass this anyway?
Even if they don't do a good job, they only need the board to work for long enough to fool the buyer.