My friend is considering opening up a online store where he would be an affiliate for sears, walmarts or kmarts. When applying for a EIN number which entity would he chose?

If he operates by himself as either a sole proprietor or LLC, he would be a "disregarded entity". He can also operate under either a "C" or sub-chapter S corporation.

If he is working with someone else, he can operate as a partnership, or C or sub-chapter S corporation.