W'Salam Saqib,
I will state a little of what I have experienced and give you some of my ideas here.
Selling on eBay is a great idea. However you should learn the basics of it including how to avoid scammers before you even partake in any sales. Best to read up whatever eBay says on this. My brother sold on eBay for a year and managed to sustain himself, without any other job, mash'Allah.
Online stores are a good idea if you can manage them. I work for a company right now that is based solely online. They sell on eBay and through their store, and most of the income comes from eBay although with my marketing the site is now getting more exposure (Alhumdulillah). If you are going to do online stores, pick a product that has forum marketing ability. Like we sell tires and rims, and there are literally thousands of forums with thousands or hundreds of members, where I can post and people will gain visibility to the site. Some are free, some are paid, but you can do a paid trial for a month and see the response, and its usually worth it.
Cell phones. You know I used to sell them, hehe. Too cut-throat, and unless you carry the higher end cellphones, and have a contact that gets them in before others do, its going to be difficult to have large profit margins if you dont have a retail outlet. The problem with cellphones is they dont have much staying power, as its an ever evolving technology, that moves very quickly. And price drops are simply huge, and a lot of money can be lost with this. I had three friends with online stores of cellphones. Only one was successful because he lists over 300 cellphones on his site, which most people cant/dont do. The other two dropped out of business. I know another guy, he has a very good contact in China where he gets the newest stuff in first, so he makes a killing on eBay. If you have the right contact, then you can make a lot. I personally would suggest another product.
Import/Export, great idea if you have the right product and reliable contacts. I have good contacts depending on what country it is you would like to import from, and what product. And my contacts also have contacts. My contacts are in Singapore, Dubai, Germany and India.
Product development is a good idea if you have the financial backing. If not, start with other peoples products.
Those are my two cents. Let me know if you have any questions for me.
I myself have started working on another side business, but its something that many people probably wouldnt want to try. Can tell you in PM. PM me with any questions if you have any.