I learned that buying collections is better than buying separately, saving money on shipping, having stuff shipped to my friends to reduce overall cost and increase profit, buying key and sought after games instead of ones no one wants, not taking payments by friends and family, meeting up with people instead of just trusting their word, not sending any more money orders to people I haven't done business with or trust and that people are more willing to buy from you if they see they're getting a deal.
For video games, I haven't counted the FMV but I'll do so and yes I want to get into cards as well so that if markets start to drop I can turn to something else I can make money with because prices of most video games have dropped.
Forgot to add consignments and finders fee are also other things that are transferable to video games.