Andrea, these are fantastic tips. As I said, I'll be writing an article based on my experiences (in about a month or so). It strikes me that you'd also want to find ebooks by the same owner (because if you find someone with one book, you want, chances are that they'd have others.
Also, this doesn't really solve the problem of finding quality works -- it's mainly good for borrowing well-known works.
Another thing. This may not apply to bestsellers, but a lot of the ebooks I am interested in costs less than $4. It seems like a lot of trouble to borrow something for that cheap (even if we assume that the ebook is available for lending and that the person responds rather quickly).