My bad. In the first place, what I meant is that you can't give up copyright - you can't say "this work is in the public domain". (Or rather, you can, but there's nothing legally preventing you from saying a year later "I changed my mind".) See http://www.rosenlaw.com/lj16.htm

However, "inalienable" was incorrect - you can most definitely sell your copyright.

Second, apparently I was wrong even then: see https://cr.yp.to/publicdomain.html

Face, meet egg.