Sorry you lost out!
That was bad etiquette, in my opinion.
My husband trades bits and spurs on some of those FB pages. Usually the etiquette would be as follows.
If someone has an item for sale or trade and is in the midst of working on a deal anyone else interested asks to "get in line". If a deal isn't made then they would be the next in line and they would politely wait until they are notified that either the item is sold to the previous person or they are next to make a deal.
I think that was rude of the seller and the buyer.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.