Well, I've had a good play around with running a shop and found out a few thing.
Even though one of the guru's said you could run a shop from your residential lot, it's very hard if not impossible. I set up a shop on my home lot with everything I needed. I had a door that I set the opening and closing hours. I had items to sell on rugs and shelves and I had my sim tend the till from 9am to 5pm every day for two sim weeks. Not one person stepped inside to even look. I went searching on the official forums for help and someone said that most (if not all) premium content only works for the resident if it's on a residential lot. Someone else said that people didn't know the shop was there and if you threw a party they would know! I'm not sure about the party idea because by the time I read that, I'd already scrapped that game file and decided to try buying a commercial lot for a shop.
You can't sell items that you salvage from the junk yard! I thought that might be a good way to make money without any out llay. Wrong.. seems sims don't want to pay money (even at an extremely high discount) for junk lol
Items that are sold, are automatically restocked at no expense to you! After my junk sale failed, I bought myself a new bed and put my old bed on a rug at my store. I set the price to 100% which meant it would sell for the same price as the catalog item - in this case $450. People were buying the bed all day and I kept seeing +450 rising up the screen, but never saw a -450 go up. I was making thousands each day! So after the initial cost of buying the items, you can't help but make very easy money.
If you have items outside the shop, people will continue to make purchases after the shop is closed and no one is tending the till! Actually, tending the till seems to be a waste of time because people will just walk up to an item and buy it there and then. You set the shop hours on the door and once it reaches closing time, nobody can enter the shop, not even the owner, but if there are items on rugs (and I presume it is the same for shelves) outside, people will still buy them.