Hi Ken, Yes, that's a very good idea.
The big problem is you have to buy land to do it (and you'd need a fairly large piece for a Superstore), and land itself is quite expensive. (eg. there's an island I'm looking at right now, which is 8912m2, and that's priced up at about £120 (real-money). Then depending on what you put on it, you may have to pay a monthly amount. The bigger the building, the more the fees and so on. (Occassionally land is given away, but no one knows until the last minute, and there's such a rush, very few have any chance in securing any).
I think it's probably more cost effective to rent a smaller area, just for a shop, or shops even (in a shopping mall, so there's more chance of Customers), to see how it goes (though this will be early next year, I would think). Some small shop areas can be around 50p per week, - I've even seen some at 14p! Everyone wants to rent near to a 'teleport' area, where most people are, and that's more expensive.
Ultimately, it may be best to just have small rented shop/s, until I'm more certain. Then if it's successful, buy land in the future. (Groups can 'club-together' and buy land (with 'virtual' money), but I'm not sure how this would work). Or it may just be easier to put some billboards up, - these vary in price a lot, but are still much less expensive than I would imagine the real thing to be.
Will keep you posted. Thanks for your interest Ken.
Best wishes, Simon.