Hmm, so I am doing something wrong then. I do list specific criteria but I end up with hundreds of PMs from people who didn't bother to read the criteria, or simply ignore them and send me their junk sites, dropped domains they've recently picked up, or link farm type pages that are way off-topic.

Point is, you can run a successful link broker website that may have a low PR but still get a lot of Google traffic.
Interesting that you say that, Prosperly. I find that the two big link brokers have vastly different fortunes with Google SERPS. text-link-ads.com is nowhere to be found for their name, whereas text link brokers is at #1 for theirs.