I understand your concern about them establishing a monopoly. Fine.

I cannot, though, understand why someone would discontinue a perfectly good Ebay business over it or why he would prefer to be paid with checks or money orders rather than Paypal.

I've been paid for stuff with checks and money orders. It's slow. It's cumbersome. It's unreliable. I groan whenever I get a buyer who doesn't have Paypal. It is a service totally worth paying a small fee for in my opinion, and I don't call that "double-dipping".

First of all, if Ebay didn't exist how would I so easily reach millions of potential buyers? That's worth paying for.

Secondly, if Paypal didn't exist, how would I have such a quick and reliable means of doing the transaction? That's a separate thing, and it's also worth paying for.