How do you know you were first?
I answered a sale ad post with "PM sent", I was post #2.
Then moments later I sent the PM.
A
Only to find out that someone had sent a PM to buy it to the seller earlier than me.
Calling "dibs" doesn't always work.
Just the way it goes.
For that matter, if I am a seller and someone who has a bad rap (not saying that about you) I might sell to someone else even if you were first in line.