Somewhat incredulously, I just learned how it works: The first person to explicitly type "I take it" on his FB page gets it, regardless of...

...whether you sent a PM five hours prior to anyone else.

...publicly replied "sent PM" to the page.

...a public conversation thanking people who tagged you, confirming this was the item you were looking for.

...watching others chime in about how happy they would be, now that the item was heading to a museum dedicated to pinball.

Thus, even if you're Elton John on your deathbed raising money for underprivileged kids, that Captain Fantastic is going to Arthur McFastfinger in Duluth because he knows the magic phrase which bypasses all manner of business etiquette which every respectable pinball buyer across the world wide web knows by heart.

Needless to say, we were surprised by the outcome and aren't terribly thrilled with it.

It's not the loss of the item, but the decision that shaving five seconds from the day by not replying with, "I have an interested buyer. Want it?" was more important than supporting a public pinball museum and an obviously interested first responder who was ready to pay full price and act quickly.

No biggie, we'll find another. From another seller.

I figured saying “I’m interested in buying it” would’ve been enough to signal that I was interested in buying it. All I needed was a confirmation... cash was ready.

An assumption was made upon their behalf that “PM’s equal negotiation” and thus we were passed over, regardless of context or who we represent (which was also known).

RPM seeks to build relationships with supportive businesses and sellers. If business is so brisk that five seconds of clarification is asking too much, we should obviously shop elsewhere.