We found out about Consignment sales in our area about 2 years ago and go twice a year now. We get some GREAT deals on clothes & gear for DD1 and we'll be looking for baby things now that DD2 will be here soon. We get a trash bag full of clothes for like $40! And we've bought DD a play tent, a tricycle and a Fisher Price laugh & learn house for 1/3 of the original price.

Here are the two in my area:
http://jbfsale.com/findevent/?n=2517
http://www.munchkinmarkets.com/ - this one is only for MN but you might find other locations if you search Munchkin Markets plus your state/city

I also know that some places call them Mom to Mom sales.