The time is not a countdown, it is the time when your listing expires.
I don't understand how your inventory is being drained, unless your inventory is quite small. In that case, there's no need to list large amounts of things using all four lots. You can just put up a small trade with one or two lots for sale.
You can
When you select what you're selling, the little box with the numbers in it showing the quantity can be selected and typed in. Just delete what's there and type in however much you'd like to offer.