Mammoth Office Furniture is a family run Herndon, VA company that specializes in buying and selling used and new office furniture for small to medium size companies.
http://mammothofficefurniture.com/
Mammoth Office Furniture has been in business since 1999 and has been serving DC, Maryland, and Northern Virginia. They have a 4,000 sq-ft showroom next to their 27,000 sq-ft warehouse. The prices are great at 30 - 70% off for gently used furniture.