Most people only look at the raw numbers. Think of it this way. If you sell a card for a 20% profit, you would essentially break even if you cashed out. BUT....if you reinvested that money into other cards and they sold for a 20% profit, your cash out percentage would actually go down. If you keep making money on your returns, then in the end you are going to only be paying a small percentage to cash out. I may not have explained that right, but maybe it got my point across.
Here is some rough math to show my point.
You sell $100 worth of cards that you only have $80 in. If you cashed out, you have $80. (you made 0% profit)
OR
You sell $100 worth of cards and keep "reinvesting" in other cards to flip. You end up with $200 in you account. You cash out and end up with $160. (you have made a 100% profit)
So if you look at it that way, you are coming out WAY ahead. The main thing is keeping your money working for you. The longer you do this, the better off you will be.