When I worked at best buy really the only games I could get at discount were used versions which ended up being some arbitrary cost they assigned to them + 5%. Occasionally when new games came in on the truck, the games we took in as trades rarely came back to our store, their cost was set very low to where I could buy them for about 5 dollars. I always checked with my GM before buying them at that price but managed to get quite a few that I knew I'd never play but could't pass up for the price, knowing I could flip them around.
Also in terms of buying you can buy the coins you need to get a game at a rate of 1 coin to 1 dollar I believe. If you see a cheapish game and don't want to trade to get the coins there is always that route. Prices used to be reasonable, I haven't looked lately, but since the seller places them they can get a little high depending on what it is.