I used to have an e-commerce business selling parts for jeeps. Spent about 2-3 hours on the phone a day, advertised on google. Income was sporadic but I had a very low overhead ($50 a month) and an average 25% profit on anything I sold. I ran the site through godaddy.com very easy to setup. I used google voice to create a (free) phone line. Never touched parts or carried inventory, I had drop ship arrangements. Now i have a full time real job (sucks) and I shut down that business.
It comes to mind that it would be a good thing to have if I was a CFI. From looking at my cfi's schedule he has full saturdays, sundays off and probably only works 5 hours a day during the week. If you had 2-3 hours a day to answer e-mails and phone calls you can setup a similar business. I stayed up until 4 am some nights just building my website and figuring out how to use quickbooks. But once I got things running, it was very smooth.
PS my average income from the company was $800-2500 a month working 2-3 hours a day. Can't live off that but it was nice extra $$