OK, this is my take on the matter.
What we do not want is a whole lot of postings (anywhere) which are basically the URL to an eBay auction listing or equivalent. That leads to huge amounts of spam which no-one wants.
I note that the initial posting here did not mention that the item was listed anywhere else. That's fine by me: I am not going to go searching through whiteblaze etc. A pointless waste of time.
> I've always interpreted "double posting" as meaning posting on more than
> one branch within BPL forums (e.g. posting FS at Gear Swap and then posting
> again at G Spot) -- which is, of course, frowned on.
Well, the problem here is not the doubling posting so much as just advertising gear for sale on GSpot. Wrong Forum channel.
Hope this makes it all clear?