2 things I think you need to do it successfully (at least from the consumer end...I have no idea about running a retail business!)
1) Inventory. If you sometimes have what I need and sometimes not, I'll just keep looking everywhere (ebay, Amazon) when I need something. I have 7 saws that I want to keep running...Husqvarna, Stihl, Echo, and Makita. Do you have all the parts I need for those? This can become very expensive.
2) Parts diagrams. Sure, i can go to Jacks or ereplacements and find those diagrams...but they provide the diagrams because it brings the traffic to their sites.
3) (I know, I said 2) Reasonable shipping. Doesn't need to be free...but I don't want to pay $14 shipping for an $8 part. Free shipping would be great...but reasonable is next best. This is why I go to those other sites, find the parts on the diagram, then go shopping elsewhere.