There's no certain anser to it because everyone will have their different reasons, but my guess would be that people don't see a point in selling their "geriatric" BPs. Either due to the mentality of "Who's going to want to buy a 20-year-old ball?" Or along the lines of "I've had it for 15 years, might as well keep it" type thing. I mean it's comparable to selling very old dogs - would you honestly try to sell a 15 year old dog? Not really. It has been done, but there just isn't much of a market for them.