i also received same risers from another seller and all fine.
Did you forget to attach the PCI-E power cables to the graphics card?
PS seller said that he know about problem,so it was low quality riser
it is used for 3 months with that 2 cards w/o risers,so i m sure that PSU is fine,problem was with riser
PS there were no problem with x1-x16 risers ,simply connected and it works