There were several rares sold on Siege recently via auction on the forums, but server birth stacked logs were not in the group, and I really doubt someone would have just dropped something like that on a vendor randomly. This is what the true rare looks like (from onlinegametrader.net database of UO rares):
It can also face the other direction. Is this what you saw?
The only thing I can think of that is stacked (as a single decorative item) and readily available, is gold bricks. They are a minor artifact, but they are labeled as bricks and look like this:
As woody says, if the vendor was selling the actual logs, they could have just fallen in the vendor pack a certain way, and each stack of logs was a separate item for sale, but they were jumbled over each other in the pack.
You can make a log pile out of regular logs by locking them down on the same tile, and using the deco tool on them or using various combinations of single logs. In a pack you can arrange things right on top of each other, though.
-Skylark