The PMA is one of the classic BW models, and has been around a long time. It doesn't have the sex appeal of some of the newer models, so it's likely that a first-time buyer might pick some newer model, and might be more likely to sell it because he really didn't know what he wanted in the first place. Someone who buys a PMA probably knows what he wants, and may be less likely to sell it. Anyway, that's my theory, and I’m sticking with it.
The 64” PMA is rated by BW for a 28-32” draw length. As Friend noted, you might be sacrificing some performance by selecting a 64” PMA if your draw length is 28” or less. However, if my “theory” is correct, you probably already know that and either have a draw length longer than 28” or feel that the stability of a 64” bow is worth a small loss of performance compared with, say, a 62” PMA.
I hope you are successful in finding one, because in my opinion they were, and still are one of the finest bows ever made.