There will be cases where this can happen. This is just like if you were selling a plane at your field and you say I want $100 for this plane and one guys says right away I'll give you 100 bux! But no sooner than that comes out of his mouth another guys says he'll give you $110 for it and he really wants it. What are you going to do. Tell the guy who just offered $110 you won't take his offer because 10 seconds earlier somebody else said they would give you what you just asked? Unlikely. I've seen this happen at my field and it goes to the highest offer. These are called "offers" and not "bids".

A guy lists something and maybe comes back the next day to consider all offers made. If one is higher than he asked why should he limit himself to only those who met his original asking price or lower? Now it is a lucky seller who has this problem but there is little that can be done about it in the real world or online. How happy is a seller going to be if he asks $100 but has offers higher than that which he cannot accept? Would you be a happy seller? Something is only worth what somebody is willing to pay for it. There is no offer/acceptance until the seller accepts any offer put on the table to them. Only then is it "sealed". We don't encourage this sort of thing but a small percentage of time it will occur either when the seller does not ask enough or there is just a buyer out there who will pay more today to get that item.

hope this explains it a bit although probably still not pleasing to you.