If you are talking about selling DIRECTV equipment such as receivers, DVRs, Genie servers, Genie clients, the answer is that you can't. You don't own your equipment, you only leased it from DIRECTV. You have to return it to them, or if they don't want it back, put it in your local electronics recycling. DIRECTV will not allow another customer to activate your old equipment, which has unique electronic identifiers.
Some of the auxiliary equipment such as remotes, splitters, LNBs, SWM switches, etc may be sold on EBay, but won't fetch much.