I've had a few people ask me about the fees for the Ebay store and how they compare to not having one. My Ebay situation may not be the same as yours, but hopefully my reasoning will give you more insight if you are interested in looking into the store route.
First, I checked out the Fee Illustrator. I played around with this at many different angles. I plugged in about what I'm averaging in listings currently (which is around 50 to 75 a month)and what I hope to in the future. I also played around with selling prices, more auctions versus BIN and vice versa. In the end I almost always came out ahead on the store. Usually, they were only off by a few dollars, but as I start to list more, I'll see more cost savings. My Ebay fees usually are between $40 and $80 a month.
Since opening my store late Sunday night I've had a few sales. I have many watchers on my items. I'm hoping as fall settles in sales will really start picking up. By the end of October I will decide to keep it open or close it. My strartgey is to list daily to build up my store even if its only 1 or 2 items a day. Yesterday, they had 1 cent auctions that store owners could particpate in. I loved that I not only got 1 cent auctions, but also with the store you get up to 12 pictures in your listings. Having the chance to put so many pictures in at no additional cost is very nice.
Please feel free to keep asking questions. I can't tell you what will work for you, but I can tell you my experience and maybe that will help. You can also email me a firstname.lastname@example.org