Well, I think the basic idea is that some owners don't always feel that way. They get frustrated with a player who's underperforming, maybe get a little nervous, and undervalue him in trades. On the other hand, I think you're basically right - it's unlikely that you'll find someone who will trade a consistent producer like Todd Helton or Aubrey Huff for lower value just because they're struggling a bit right now. So I think you're right - a buy-low player doesn't just have to be a player who's underperforming, but also one who's unproven enough that his current owner is worried that he'll continue to play poorly.
With that said, here are a few guys who I think are potential buy-low candidates. Keep in mind that the fact that they are buy-low candidates means that there's some risk involved (often, this risk is simply that they're not proven and have only had one good year), and there's a distinct possibility that they'll continue to play poorly. That's why Todd Helton's not on this list - it's going to be pretty tough to convince a Helton owner that he's suddenly going to hit .280 with 15 homers this year.
Carlos Delgado (risk because of new ballpark)
Ken Griffey Jr.
I'm not necessarily saying that I'd go after all these players. (I'm not really expecting great things out of Cabrera, Rowand, Bay, Estrada, and McPherson, to name a few.) But basically, these are the guys who were expected to perform better than they've been performing and who also have sufficient risk attached to them that their owners might be willing to sell them at a lower price.