I sell on Amazon part-time since writing is my main business, however it is easy to do. You can sell two ways. One is by storing the merchandise at your home and shipping yourself when it sells. The other is called FBA, Which stands for fullfillment by Amazon.
I choose FBA because they store the product in their warehouse and handle the collecting of money, shipping, and customer service. They charge fees for this so a good rule of thumb is to send products that you can sell for three times what you paid for them. For example, you pay $5 for a game you found brand new at a thrift store so you need to able to sell it for $15 on Amazon to give you enough money to pay their fees and still make about $7 for yourself.
Ranking is also important depending on what category you are working in. If you decided to start selling just PM for a ranking sheet that may help you out.
I sell used books and toys and household good mostly. If you are short of money I suggest start off selling books as used books are cheap and some can be sold for over $100.