I think you're right, I might as well just email a bunch of offers around rather than putting them on the site. I don't get any lower response to email than I do to offers on the site.

I too get a lot of offers that don't consider (or don't do it well) what I might be looking for. I'd rather get the offers and start a conversation than not get them, so I ususally try to decline politely, offer some ideas about what I am seeking, and suggest some possible trades.