Okay, several reasons for me;
1) It is
way to small to put an actual advertisment in there for an item you're trying to sell
2) If I'm trying to sell something, I want to sell it as quickly as possible. That means as much advertising as I can get. I don't spam towns, but I do give one or two little "WTS/WTB..." messages before I switch districts, and always after my last message diappears. So on average, I probably spend about one minute or less in each district. Point is; I would love to use the search function, but, as has been pointed out earlier, there is no way to fix a message when you travel. Even in the same town between districts. Let's see.... *Holds up hands in a balancing motion* Make a woefully understated message in search and then spam one district for half an hour until it sells or I get bored
OR Travel between multiple districts of multiple towns reaching a much, much greater populace in a shorter time, limiting my spam to one or two messages in each district.. hmmm....
3) As far as partying goes, you're either going to get one in a few seconds... or your not. Personally, I don't want to wait in a town long enough for a party that my search message would even be noticed.
I think the main problem with search is that it just doesn't have enough popular momentum behind it. For nearly two years, players didn't have it, so they never got the into the reflex of using it. New players don't see a lot of people in there, so they think, correctly or not, that there isn't very much exposure for it.