you have to sell a qualifying product. I used to work for a microsoft reseller back in the day, and I sold OEM copies of windows with 3 dollar case fans.
However, you must bundle the OEM with something, you can not sell it alone. Technically you could sell it with just about anyting I think. Our system would not put in the price break for OEM unless you also had a piece of qualifying hardware on the same invoice. The cheapest thing I found that worked was a 3 dollar case fan.
You also get no support either. With retail boxed copies they usually give you 30 to 180 days support depending on the product/company. With OEM you get none.