My family has gotten back into playing Settlers this year and has really enjoyed it, so I started looking for expansions that would match our base game, that is 3rd edition.
I was surprised to find that 3rd edition expansions, specifically seafarers and cities &knights are basically non-existent either new or used anywhere that I can find- BGG, craigslist, ebay, amazon, etc. I know it's a popular game, but its also a mass produced game that was very widely sold, so where did all the copies go? Are the more recent versions so crappy that everyone with an old version is unwilling to let theirs go?
People would probably not just be selling the 3rd edition expansions. You would probably have better luck looking for the 3rd edition base game + expansions as a set.