I received a bad part. What should I do?

i keep all my pms on here intell the deals done ive done two both went well i sold some thing and got some thing i tryed to tell the guy the i soold to i didnt know how to check some thing on them he said thats ok ....... now if that happen then its ur bad ........now say i told him i knew what i was doing then he got them and i didnt then its my bad would have gave cash back .... but ike i said i did a good job telling him what was what i would like to here from the seller on this i would give him a week or so to do some thing then call him out right here so we can all poke him lol