Well, the only way you can get things like xboxes, it to have an extremely large buying budget. Places like walmart and such buy hundreds of thousands of units at a time. Especially in high ticket items such as electronics, the profit margin is very low, and makes it almost impossible for someone just purchasing a few units to compete.
Too, places like Sams Club, Walmart, etc., don't have to make much of a profit on any item that they sell. The reason is two-fold. The first is that overall they are selling a large number of items and just have to make a small amount of profit to stay ahead. Secondly, and most people do not know this, a good many large and small retail stores charge what is called a 'shelf fee'. A shelf fee is a fee that a company, such as microsoft(xbox) pays the store for letting them sell their product in their store,(walmart).
Most large retail stores have a MINIMUM shelf fee in the amount of $50,000.00. And this fee is charged per product (xbox,xbox 360, etc.) So walmart is going to make some sort of profit simply by allowing xboxes to be sold in their store. Then they will also make an additional small profit on per unit sold. Thus, they can afford to sell them much cheaper than you will see at small 'mom & pop' outlets or websites....