Okay.  So forget the gift cards, you can't compete on price if you are marking them up when other retail locations don't.  The trading cards should probably go on e-bay or direct to a sports memorabilia or card shop.  Research for the stuff you have that is valuable enough that you know it will sell and forget the rest.  You can't compete with dedicated retailers who buy at wholesale and less common cards have a million different people undercutting each other in the used market.

What kind of games are you selling?  It sounded like Warhammer type stuff?  You are again competing with established retailers.  Even a local comics shop that doesn't have a particular item will gladly be willing to special order it from a wholesaler, they do it all the time. 

You don't really have a business here, you have a garage sale at best.  I think your management courses may have focused a bit too much on managing employees and customers, the human relations type stuff, and not so much on the economics of running a business.  That's still a very valuable skill, you just need to know where to focus it correctly.  I probably have the exact opposite problem, I'm homeschooled and my dad is an Agronomist so he was great at teaching me about economics but I haven't had much experience socially to work with customers.  I think you should just keep your collection of cards and start looking for some management jobs at a small business so you can learn from someone with more experience at working with wholesalers and distributors.  Collectables like that have value beyond money, I would never sell off my collection of ponies.