Ebay is a lazy, expensive and not trustworthy source.
This bit. I have had excellent success on ebay for a number of things. The Japanese sellers are especially good. One just needs to know what one is doing....
Sources far better than ebay include PentaxForums, B&H, KEH, and many dozens of other direct sources with strong reputations. Often, local Craig lists are also strong.
I would say that KEH is excellent, as are the Pentax Forums. I think B&H and Adorama trend high for used. Don't know about "dozens" of other direct sources, unless you mean the Japanese ones----that sell on ebay. Craigs List can be hit or miss.
Depending on the particular lens desired, and willingness to look at third party brands, the successful search might involve some patience.
Yes, exactly. If you are serious, then checking at least daily is the way to go.