Does having an extension other than .com hurt your search engine ranking?
It obviously doesn't make a difference in PPC.
The only example I can think for not using a different extension would if you wanted to advertise your domain on the radio or something like that where people would have to remember the name.
I own two such domains for small sites and they do fairly well.
The com site is community stuff; chat and message boards, ie people content. The .info site is where all the articles, faqs,calendars stuff is.
While it's taken a few people a bit to even recognise .info is as domain, I notice when I speak to people live, they recognise the brand name and often say "Oh, you have the information site", which delights me to no end because it's clear the perception is the info site is industry-wide and they are more likely to pass that on to a friend than just the community-based com address.
All the features of both sites are linked, but I don't maintain two copies of anything. If my chat server goes down, the other content site is up. Now I need to set up some way of finding out how people bookmark them though.
I know none of this addresses the original question about buying and selling, but thought maybe info on how people use them might shed some light on the market for .info domains.
~Dian