Currently at Amazon.com, Simon & Simon is at #1,999 in terms of the most TV and movie dvds sold, and Diagnosis Murder is at #2,471. Magnum P.I. season 5, a show considered similar to Simon & Simon, is at #555, and that show appears to be scheduled for completion.
Note that movies dominate total unit sales over TV in general, so that isn't saying Simon & Simon is necessarily doing poor. Also, all three sets have been out a month (not sure about Diagnosis Murder), so their peak sales perioid has passed. When you get down the list, I'd imagine a few more units ordered can really push the sales up the list, so the numbers can fluctuate quite a bit day to day.