It depends on what you re selling. If you just want to clean your house and are selling your un wanted items then you ll make some money but its not like a job. If you want to make a job out of it you ll need a flow of desirable product that you can sell at a better perceived value than the competition than the competition. If you have some IP component to your product and its value is easily apparent e-bay roll out might be the easiest avenue. But over riding all of this is the fact that making a sustained profit is not an easy task.
No, from the people I know that use it, they make a lot of money off selling little trinkets. For example I know someone that bought an old fishing reel for $5 at a yardsale. He never used it so sold it on eBay and started bidding at $3. It sold for $15!
Not really! Just get the items cheap from whole sellers and sell for just under the RRP and people will lap it up. You will be swimming in prophet