I've read through this and I have to say that I'm a little confused...if there is an ad in the classifieds that tells me set XYZ is for sale, mentions terms etc, and I see that no one else has posted (very rare) and this XYZ looks like what I'm looking for, then I post a dibs, or, PM coming, then I follow that by immediately sending that seller a PM letting them know of my interest in XYZ. I will ask them any questions they I may have, then I wait for their response. I guess you can call me a social retread, but, seems like I'm an online forum that is how someone should go about procuring the item the seller is selling. I feel like I'm on pretty solid ground here, so far. When it comes to the other part of this op, how else am I supposed to express my interest in an item of I don't speak up, in the instance of a set that may get split? It's not like is an uncommon thing, sets getting split. What's the issue on stating your desire for a rack or barrel, of whatever part of that set that's being offered? I then, again, send a PM letting that person know of my interest in that item if a split should occur. Doing this let's the seller know where the interest level is, and, also let's others know, which is useful if the seller is adamant about not splitting, so someone else can guage interest in case they end up buying the complete set to get the one rack they need and then resell the rest.
How else can a person get in line if they don't post, and then follow up with a message to the seller? Also, this can help with having a record of who asked, and when. Most sellers I've dealt with have shown a great level of integrity, most transactions I've done on here have pretty much gone this way. 99% of the sellers here have a been great to deal with.
As for people dibsing (is that a word) they have a responsibility to be responsive, if not, too bad. If you want it don't waste the sellers time.
I guess if you're getting bent about people posting their interest in items then you may have other things bothering you.