For what it's worth, that buyer - who seemed to have ghosted me - did eventually respond. His explanation was that he "thought it had been taken care of", which seems a little mysterious, but I learned long ago that email/PMs are easily misunderstood, just due to differences in the way we express ourselves and word things. By the time he responded, I had lowered the price of the filters; he said that he would pay the original price, and that payment came through ptomptly. So he might be a little disorganized, but he's an honest person in the end.