Think of Ebay as a type of real-time stock market. It's pretty easy to search for something and determine the average price based on all the listings, and since everyone prices competitively that average price may tend to spike or lower overtime for one reason or another . You shouldn't price your item above that average price unless there's something unique about it from all the others (signed by a celebrity, mint condition, etc.), you also don't want to list the item too low either otherwise you'll just be letting the buyer take advantage of you (unless you really are just looking to get rid of something with no regard to your profit margin).
Speaking of profit margin, you didn't give many specifics of your business plan there but from what I could gather it sounds like a complete waste of money/time. How I understand it is you're going to buy your stock from somewhere (used goods stores, or even ebay itself) and then turning around and reselling the item on ebay for around 20% markup of what you bought it for. Assuming that's accurate, unless you have some hookup with a large amount of goods that will sell to you for dirt cheap, your plan will completely fail and backfire in your face. Even if they're retro/rare games, without some kind of hookup no one is going to sell you a cheap enough copy for you to markup enough to make it worth your trouble—they want a profit too, so they're just going to sell you the average price of the game, no matter what it is. Since you can't (easily) sell a game for ABOVE the average price people currently pay for, if you pay remotely close to what you sell for then its a waste of your time.