I live in NoVA and there are two retail locations I know that are left in the area, although I'd be interested if someone knows of more. There are:
Maryland Stamps and Coins in Bethesda, which are APS/ASDA. Run by David Most. The few times I've been there they've been helpful.
There's a place in Rockville on Rockville Pike in MD. I haven't been there in a while and its mostly devoted to Coins. However, they have a stamp specialist from whom I have purchased starter collections for my children from boxed lots. The store is probably "Coins of the Realm".
However, and noting I haven't seen the collection, you should expect any retail dealer to offer a very low appraisal or nothing. So if you choose the route of walking into a dealer's store and asking for a valuation, expect the worst.
There are also clubs in the area, though I don't belong. The Ayr Hill Stamp Club in Vienna (my home town) used to meet at the Patrick Henry Libary. Perhaps it still does. Here is the website with meeting dates, although I don't know if this is 2017 or some past year:
http://www.angelfire.com/va2/ayrhill/