I’ve gotten some good deals on lightly used 24" dishwashers. I’ve never seen an 18" come up, though I’m sure they’re out there. I think I paid about 50% of MSRP on my current one that has been very good to me.
I see lots of people saying there are plenty of 18"s, and I’m sure that’s true if you buy them new. I guess I just wanted to agree with you and share my experience.