You'd be shocked at the old sets you can find at toy stores in Europe, Asia, and South America.
In May 2006, I walked into a small toy store in Thessaloniki, Greece and walked out with, among other things, a 6396 International Jetport (from 1993), a TECHNIC Rebel Wrecker (from 1994), the old lights brick set, a few old grey roadplates, and some other small town sets. And that was just one toy store. Small toy stores in Europe are filled with old LEGO, Playmobil, and other toys that have been sitting on shelves for years. It's even better in Argentina, where the toys are so expensive that they just sit on shelves forever. You can got a toy store in Buenos Aires and find sets from the ten-year old LEGO sets without much difficulty. Italy, though, is the best place to find old toys. Hence, you'll notice that a lot of eBay sellers selling so many mint, old sets are from Italy.