So I was selling an item at Jita in bulk. The situation was roughly like this:
$2000 | Quantity: 3 | Neat Item <- My sell order
$4000 | Quantity: 1 | Neat Item <- Someone else’s sell order
$4250 | Quantity: 1 | Neat Item <- Someone else’s sell order
I got a notification that my sell order of Neat Item had gone from 4 to 3. When I looked at my transactions, I saw that the order had gone through for $4250 even though that was not my sell order.
If it matters, of the three sell orders listed, mine was issued before the other two.
It can also be for convenience. For a module that is traded widely, with small differences in price, in a trade hub I may purchase from the seller that has the quantity that I want available, rather than making multiple, but cheaper, purchases.
I do this quite often for modules that I buy in bulk. I consider my time more valuable than the small difference in price.
So, in a lot of instances, sellers benefit from this. But, I consider it worth while, as my time is more valuable (to me) than my isk.