I’ve actually had people accept both, under and over (but more often over). I think it depends on the condition of the bag, how much I want it, how long it’s been listed for/whether it’s been relisted multiple times with no one bidding, what I decide to offer. I don’t worry too much about the seller getting annoyed (is that just me? I feel like they should know it’s part of what goes into selling on ebay, plus they can just decline the offer anyway...), and go with what I am willing to pay for a bag but always knowing there’s a chance I won’t get it.