Check out the Washington DC Craigslist - people are always trading metro cards for pretty much anything. I've got no idea how they get them (maybe they are Metro employees stealing them?) but they're always available. Also, I think if you buy a 20.00 card, you automatically get 22.00 worth or something like that. Dollar value cards don't expire, but they can lose their magnetic readability as they get aged & unless you go to Metro Center to get the money transferred over to a new card, you're pretty much out the money on a card.
You can buy multiple day cards here:
http://www.commuterpage.com/products.htm