What does this mean (if anything) for region-locked movies - can I legally break the region lock (or buy a player that does it) so I can watch a movie I bought overseas?
Not comparable issues at all. This opinion is only about re-selling items.
In your implied analogy, the book is the DVD, the re-seller is a re-seller. This case essentially says you can buy a Region X DVD in country X and re-sell it in the States provided that the Region X DVD was legally made. So this means if you buy a legal Chinese DVD in China, you can sell it in the US. It does not mean you can buy a pirated DVD in China and sell it in the US. It does not say anything about region unlocking.
You should always read SCOTUS cases narrowly. Anything that the decision of the Court does not rest on is called "dicta" and has no controlling authority, but merely persuasive authority. This means if the decision of the Court does not rely on a statement in the opinion, then that statement can persuade other courts, but it doesn't instruct them to do anything.