Let me start by saying that all of my GLO insignia is very precious to me. When I clean out my closets and make a pile to donate to Goodwill, I deliberately hold back my old sorority t-shirts because I don't like to think of a non-member wearing them. (Then I feel kind of guilty, because I envision someone who really needs a t-shirt having to go without.....) In short, I can't imagine selling my pin on ebay or elsewhere.
That being said, that pin is just a hunk of minerals. It's beautiful, and it's very meaningful to me and other members, but wearing it does not make you a member any more than wearing my old convention t-shirts would make you a member. In the final analysis, the membership pin is an inanimate object.
If one of my sisters is desperate enough to sell her pin to get a few bucks, my heart goes out to her. And if some non-member wants to throw their money away by buying it, .... whatever. There are many more important things in this world for me to agonize over.