That is a very big question that deserves a very long answer.
My guess is that there are several stamp dealers in the Baltimore area. Without knowing anything at all about what you have, I would suggest you get one of them look at your material first. Depending on the quality and quantity of your material they may or may not want to make you an offer.
Without getting too long with this reply, I would say the next best thing would be to try to find a local stamp club with an expert that would be willing to help you.
The hard route is to try to value the material yourself using a catalog like Scott's. This can be dangerous, though, because there can be stamps that appear to be alike but be worlds apart in worth. Again, without knowing what type of material you have that's about the best I can do for you.
I'm sure some others will be along to offer their advice also.
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