Really? 'Cause if you sent a PM to me, I already know. And if you sent a PM to someone else... why do you want me to know? And just so you know... I don't care. That is all.
I think that people do it in the sales forum to gently let others know to back off because it's going to be theirs soon or a kind of dibs.
PM = Proprietary Mark Old English for "stake a claim" or the early germanic, Die Uhren sind Mine! Best summed up of course by the American slang, DIBS
If you're selling a watch, you probably come and check the thread for comments. I've known sellers that don't notice the little icon at the top right that signifies a message, but will look in their thread. Also, it's easy to tell who replied first in the thread. PM's actually list newest first, so you'd have to go down the list to see the first reply - a fact I once forgot about when a couple people sent a message saying they wanted to buy the watch. Answered the top message (the newest one) and had to tell that potential buyer that I screwed up because they were beaten by a few minutes. I felt bad, but had to be fair about it.
I thought it was you sent the seller a pm to say you would like to buy? That lets others know that they were beaten to the watch.... @Darlinboy like your sale here https://omegaforums.net/threads/1967-bulova-wrist-alarm-outstanding-condition.26260/ I was sending you a PM and then would have put a pm sent on your FS thread. In which I returned to see @Tritium had sent a pm Grrrrrrrrrr
I sent one a PM and didn't got some replies. So the watch didn't become mine. Next time I will PM, and say did, maybe twice.