For the dress, you can also try oncewed, tradesy, and preownedweddingdress.com. I've been looking for a "used" one and those are where i started.
idonowidont offers some pretty good customer protection - the diamond (if there is one) has to match the specs you advertised - so that is the only place we really considered buying a diamond from. They do take a certain amount of commission.
Also, if you have a designer setting (Tiffany, Verragio, Tacori, etc), you might consider selling the stone and setting separately IF you can find a buyer for the stone that doesn't really want the setting. We saw several settings that might have worked for me at a steep discount on idonowidont, except they were all platinum and i wanted rose gold :/