I am a newbie in SL, with 10 years of designing usable web pages and 10 years before that in retail sales. Here are some thoughts about SL stores. They are all based on actual SL shopping experiences.
1. Don't make me work to buy something from you, unless selling is secondary to playing shop owner. If you want to sell things, help me buy them.
2. When I teleport to your shop, make it easy for me to know I'm in the right place.
3. Don't make me fly around to find your shop or sign.
4. If your slurl takes me to a market square, help me find your shop.
5. Have a sign at eye level telling the name of your shop and what you sell.
6. Unless you have a tiny shop, give me clear instructions about how to find things.
7. Don't have a dog or scary creature. I like animals but I had bad experiences with dogs as a child. If you have a dog, I am gone.
8. Tell me how to buy a product. I looked at a lot of hair and could not figure out how to buy it. (Except for the demo's.) I finally had to IM a store owner.
9. Tell me everything I need to know to decide whether to buy your product.
10. Try to have products at eye level. If you have them higher, make the signs readable from the ground.
11. Do not have huge open spaces. I am not here to exercise my mouse finger.
12. Have people who don't know anything about your shop teleport to it and try to find things. Do not help them. Do not say anything. Just watch.
13. The customer is always right, even when they are not. Do not insult me.
14. Decide whether you want to play "store keeper" or actually make sales.