I'm not really sure how to answer what you are asking for. This has happened when I'm selling via the barter window to folks with various tradeskill component lines. There's only one line for the item for the person I'm selling to. I mostly do this in PoK or PoTranq (basically selling stuff that folks have sold to straight to vendors). I've never seen the reverse happen like Laronk says. It's always I'll start selling a block of 20 items, but the transaction will only take 2-16 ... so my barter sell window thinks I'm selling 20 items, but the game only processes a lower amount. Eventually the number will go up and if I'm selling enough items (like 200+), I can get it back to the 20 items per sell. I'll spend some time this weekend on various servers and see if I can get it more concrete (like timestamps etc). This has happened only on Live servers, but at least 7 different ones.