I was a financial advisor (stock broker) for 15 years and now sell cards full-time through my LCS and on eBay. Making it in this business is the same as making it in any business.
1-You have to have capital. So many "players" in the card trade have very little actual bankroll.
2-Understand the time value of money. I would rather sell a 5k deal in one day than sell that lot for 6k over 3 months.
3-Have business sense. Understand debits and credits and expenses and income. Yes, there is a difference between sales and profits.
4-Know your business. If you are a lawyer or doctor or car salesman, you have to know your market. There is no difference in the world of cards.