I am speaking from my eBay experience over the years. I remember the early days, I was losing money, getting scammed, breaking an eBay rule which would get my account suspended and as a consequence I would lose some money and time too. I have tried selling almost anything ( Mobile Phones, PC accessories, clothing ) I targetted many categories until getting settled with just one category in which I specialise now.
As years and months went by, I finally settled to one account and one category and tried finding the cheapest possible supplies and set daily/weekly/monthly sales targets. I have noticed each month my sales have increased so has my profits. Bottom line is it takes some real action before you see money, your losses in your early days because of inexperience should not let you down.
Just do not hang out with losers, they'll make you feel making money online is impossible and that is a bad bad way to start.