People post on this forum when they have aircraft for sale but you can also check at the end of Lagoon drive at the gate, people post notices there as well. I’ve included a picture of a poster that I recently took for a Cessna 177A for sale.
Sorry for the delayed response, but I’ve found the best place to find aircraft for sale in Hawaii is Craigslist. I never cared much for Craigslist, due to the amount of stolen goods, and other illicit activity around it; however, here in Hawaii, it seems to be the place with the most activity. Personally, I wish more would use this forum, as it is much more legitimate, and doesn’t get filled up with miscategorized stuff.
Anyway, Craigslist: I hate it, but it seems to be the best place to look in Hawaii.