Buying used ball used to make good sense. You could get a decent ball for $20-$30 bucks, shipping was $10-$15 more, and a full plug was another $15, so you could end up with a really good piece for around $50-$60.
Now, people are asking $50-$75 for a good ball, another $30 to ship it, and another $75 to have it cleaned/plugged/de-oiled/re-drilled at the shop. You can tie up $150 in a used ball really quick these days, and that just isn't worth it when you can find great deals on NIB discontinued stuff and get a brand new ball just as cheap or cheaper.