I encountered a similar issue recently. It’s awkward. Not sure there is a perfect answer to your question. I didn’t find one to mine.

look at the time in the message.  that should tell you who was first.

First I’ll take it with no qualifications.

I saw this post, and then after looking at the OP's posts, saw that I was the guilty party in this case.

I saw an ad yesterday, just posted, for a high wall action.  Seller said PM's first.  I sent a PM (for this item, and another item he also had for sale) and also did a post in both threads, trying to forestall exactly this from happening.  If I had just sent the PM, and someone else posted, then it would appear the seller sold to someone other than the first respondent.  

In this case, there was no post before me when I typed in my post and hit send.  I didn't see the other post, truthfully until I saw this thread and looked.  My post, and RJ 35-40's, were literally posted simultaneously, from the standpoint of one or the other seeing the others post.  We were typing at the same time.

Not what I intended to do, cutting in on something someone else was trying to buy.  If he had messaged me that he thought he was first, I would have deferred to him.  If the seller told me someone else was ahead of me, I would have obviously deferred to them.  

My apologies, RJ, was not trying to beat you out when you were first, I just followed the instructions to PM the seller, and then didn't see your post after I posted my message that I would take it.  I've already paid for this and the other item from the seller.

I always post a reply stating the I will take it IMMEDIATELY followed by a PM. Just covering all the bases.   

BTW,
That next DST high wall action that anyone posts.  I will take it!   

Just Sayin'

Jack

This has happened to me several times.  I post the first I take it and then informed I am not first.  This sucks.  If you can’t openly pick an item in open with a time stamp for everyone to see, then you do not deserve consideration.

Allan

My preference would be to give the option to purchase to whomever replies to the post with “I will take it”. I see no problem if that person has a question or wants to see a photo especially if no clear photos are in the original post. Making a lower offer though would void the first right to purchase.  Vall’s example should be the way to handle the transaction. An immediate PM should be sent to verify the request.  I do not like the idea of only requesting a PM as it leaves others with the impression that the item is still available.