I say buy.
1. If you're going to sell pieces like Beckett, Lester, etc, at least give them a chance to build their trade value. There's no sense in selling them at a discount because of a poor season. If you want to trade them during the offseason, I'm all for it. But for now, you're not going to get much of a return on either until they show they can still pitch.
2. Call me crazy, but I never count this team out until it is mathematically out of it. If we had a bunch of pitchers with s***** track records, fine. But this team isn't playing well because of underperformance, not lack of talent. And I will always debate that talent doesn't just disappears at the drop of a hat.
So, I say buy, but if you buy, go out and get a guy like Justin Masterson or Josh Johnson, not a rental.