No if nothing sells he just has to be the initial charge $1 On resell quantity does not take priority as he has already paid for that. He is only renewing the duration.
Yes if a quantity sells he does get an added benefit of boosting his duration. A benefit for the user for paying you extra for quantities.
This all makes perfect rational sense to me as it takes into account both the admin of the site as well as the user. You charge a fixed cost for a duration on a category. A user pays the fee. If he happens to post quantities you get the added benefit of more cash upfront and the user gets the benefit of his quantities he paid you for if one sells of boosting his duration cycle.
Now if nothing sells he has to be like every user posting one ad and renew his ad for your single fee. He has already paid you for the quantity already before.
I think this is probally the most fair system of doing things.