Unless every single publisher in the world blocks used games, then you would be able to trade in certain games. Besides, why would Sony come out and say "we're letting it up to the publishers, so you MIGHT be able to play used games." They designed their system to allow the use of used games, so that's what they're saying. If publishers want to use a system that eliminates used games, why should Sony take bad publicity? With Sony's stance on it, if publishers do it, publishers get all the bad publicity.

I'll post this one more time:

UPDATE: More relief... At a roundtable this morning, Sony's game studios chief, Shuhei Yoshida, told reporters that any requirement for users to register a game online in order to play it would be left to game publishers. Sony won't require that.
If publishers want to block used games, they 100% absolutely can.