ok, i sold a gpu on ebay, with no returns but they returned it anyway. when i sent the gpu out i could get 30mh tweaked. when i got the card back the most i can get weith the rx 580 is 15mh has somthing changed in the etheruem or software or is my card now slightly burnt out?
Guessing you have tried to flash the BIOS with the one you use to get 30MHs?
Did you make a note of the serial number of the one you sold?... maybe the buyer returned the same model but different/faulty card.