The only time a deal should be final is when a seller accepts a price, and a buyer commits to buying it, no conditions attached. You can always buy it, and if it doesn't fit, sell it on in your own time. It is really time consuming (at least for me) to sell things on the hub, so to turn down people that are willing to collect cash in hand at my door step, only to go through a whole rigmarole with the first buyer that said dibs, is too painful. Most of the time I would have rather just not sold the item to begin with.
