An online shop is very much the same as a real shop, you need great products and good prices.
Being online, people tend to expect things a bit cheaper so where i come from, Australia, rather than competing on cost, i tend to push other issues:
1. Superfast postage
2. Guarantee of quality and good warranty
3. I try to provide excellent service
4. I include a very small, FREE, gift with every sale. I sell darts, so generally include a free set of flights or shafts.
5. I also enclose a business card with each sale, but instead of the normal stuff, it has a HANDWRITTEN thank you for choosing to buy from us....blah, blah, blah. I try and become a "friend", not crawling, simply a person, a REAL person behind the shopfront.
6. We started a customer rewards point scheme to offer savings to repeat buyers.
One more VERY IMPORTANT thing is i especially target and aim for people in remote regions. There is no way in heck of them being able to walk into a dart shop, so i figure get them to buy once, treat them well and i pretty much have a customer for life.
Just some thoughts that may, or may not assist you