If there's no window and no other source of natural light, then according to the MLS rules it's a 1Br with a den. That will impact market value if you are competing with other legal 2 Br's. I often see such units marketed as 2Br's but when compared with true 2 Br's they often loose out due to potential resale issues. As Michael says, it's all down to price which ultimately shows you that if it's priced right a buyer may not care if there's not a window. However, "priced right" means dropping your price until someone is prepared to buy. That's where it will affect your resale value.
Good luck, Ken.