Ebay
I am sure you are aware of ebay- it is the worlds largest auction site. Thousands of people have abusiness on ebay and make money out of it. You can do it too.
The most important thing is to gain familiarity with ebay and to understand how it works. There are details of this on the ebay site. After this, you need to register with ebay. If you are going to sell things on a serious level, then you need to get a paypal account. Paypal acts as a secure means of transferring money from the buyers bank account to yours.
If you are mistrustful of ebay or any other online sites, it is a good idea to set up an account with a debit card, with a relatively small balance in it, and with no overdraft, so if by some means your account is hacked, then you have limited your losses.
Start by selling a few old things that are lying about in your home. Once your are comfortable with the sytem you have 2 options.
You can either make money by: Buying a product externally and selling it on ebay
Buying a product off ebay and selling it at a higher price on ebay.
For both of these methods the product is absolutely vital. There has to be a significant demand for it. You should also be aware of the various fees on both ebay and paypal, and counter these in when deciding on suitable selling prices.
Once you have found a niche, it is relatively easy make money off ebay. I have only just started, yet make a reasonable income off it every month. I amk constantly learning, and as you learn about it, you will make more money from it.