I sell on three ids, one is a store which has 200 free listings then I have 2 non store ids one of which is used for auction items and the other for BIN with best offer.

If you are going to be listing one product where variations are available then I doubt if a store would be cost effective, if you listed using a non store id you could list for 10 days and then just relist every 10 days adjusting the number of stock you have left. As you get up to 40 listings free you could sell a few as 5 day auctions which would keep your items near the top when people search by newly listed or ending soonest.