i don't believe their worth it. i believe that end-users aren't typically found shopping around on Sedo. i believe that most potential end users have to be contacted, and sold and persuaded on that domain name. and the seller must do a pretty good job at it, for the value of domains (and it's advantages to indexing/brandability) is still not known to the mainstream.
also, it is a form of mass-market advertising to other domainers, which i don't believe is the appropriate form of marketing when finding end users