No offense, but this is wrong and oversimplified bad advice for someone new to the hobby. You are using the term “flipper” instead of “selling”.
A flipper is someone who immediately sells (flips) majority of what they buy. The people referenced: myself, Gary and Spencer couldn’t be any further from that pattern.
I agree with this. There were probably many times over the years where you could have sold a card for a 25%/50%/100% profit, but you continued to hold on to it. A flipper would sell as soon as they could get a decent profit and then move to the next opportunity.