How about I sell an extra cable I was shipped instead of returning it where it came from.
Why not just keep it as a spare?
I have plenty of cables myself at home
If they screwed up and sent it to you, its yours to do with as you wish.  If that means selling it, I say go for it!  It's not like it was super expensive or anything.
I never use the cables myself.  But I'll sell it and buy 2 new ones.  Also, I'm thinking of making a sale on amazon
I think it would be okay to keep something that was sent in error. Most places will tell you to just keep it anyway. You can go ahead and do what is best for your situation.