in my opinion it's not really profitable for the amount of work you have to put in.........
you can guess at prospects and rookies and then sell when they are at top value, but? you will miss alot trying to do it............
alot of people have these stories of cards and how they do this & that...... I really don't believe that anyone makes "ALOT" of money on cards..... you can make some, but I think most people end up in the red with ebay & paypal fee's, buying product etc. it's really not a smart biz to be in, if you operate it as a hobby and then have a regular job it's alot better because you have steady income and don't have to worry about it from check to check.... you will get desperate and sell stuff for a loss time to time if you need money.
Collecting Carlos Correa / Byron Buxton / Manny Machado / Bryce Harper / Mark Appel / Kris Bryant (lower end stuff & bgs slabs) Basketball: Blake Griffin / Derrick Rose / Paul George / Damian Lillard Football: Andrew Luck / Russell Wilson