Eileen Flieg from DaffodilCorner says
If I understand correctly, this how it will work once the new listing feature is available to you:
You have 10 mugs. You create one listing for the mugs, with a quantity of 10. You are charged 20 cents.
Buyer A purchases 4 mugs. The listing will now show a quantity of 6. The item itself is not renewed. It still has the original expiration date. You are charged 60 cents in fees, representing the cost of the listing fee of the additional 3 mugs you sold that you hadn't yet paid a listing fee for.
If I've got it wrong, please correct me.
When the buyer purchases 4, you will be charged 80 cents in listing fees (60 cents for the three listings which were not previously charged, and 20 cents for the newly renewed listing).
The listing is renewed (with a quantity of 6), with a new expiration date of 4 months from the time of purchase (it will behave as if you manually renewed the listing).