How do i sell my miles to other people?

Well, the only way you can do this is by booking a ticket on someone else's behalf. This wouldnt fly very well if they see you're doing this too often.

So, personally, if flying isnt you thing, I wouldnt rack up miles - cashback would be an easy alternative

Strictly speaking, no selling is allowed. So don't do it, or the airline/FFP have the right to shut down your account.

What you can do however, is to redeem on behalf of someone else, and then whatever compensation from the person you redeemed for is up to you to decide.