Plagiarism is stealing, bad.
Self-plagiarism...re-copying old stuff... not so bad IMO.
Self-plagiarism is an oxymoron. When you sell your writing, your work-product is your words. If you sell a work to one publisher they have exclusive rights to it. You cannot sell the same exact content to another publisher.
Everyone repeats themselves, and the flow of paragraphs in different works by the same author may resemble previous work. Exactly copying your own previous work and reselling it is fraud.