I know you are/would like a cut and dried rule.....but...
The - "I'll take it" - is really just a standard way of saying "I would like to buy this knife-if it still available."
And most of the buyers here know this I think?
Some of the PM sent messages - are asking questions and maybe even hitting the guy with a low ball offer. Which does work some times? And they do that instead of posting - "Hey I'll give you this much right now for it?" in the for sale thread, which everyone else could see. And the seller may reply back privately that it's a deal.... instead of in the thread. So that trumps the I'll take it post.
Also - Bladeforums is NOT the only place these guys sell knives.
Makers will post their work on their own web sites and other forums for sale.
So even though you are the FIRST to post I'll take it - before any other post are made.
Someone else may have got it else were first......
And most buyers and sellers here know this - I think?
Knife makers - and I'm sure collectors too - want to move their knives FAST.
Because they like to eat......
So this is not going to change -
Edit to add - - Over the past nine years I've been lurking about here.
The I'll take it post works MOST of the time...... but like you have seen - not always.