I generally try to sell locally (canuckaudiomart) but had a couple items not long ago that I figured would do better on Agon. What a pain-in-the-*ss it was setting up an Agon listing vs canuckaudiomart (and as others say, they make it hard to communicate with the buyer).
One thing that surprises me in this thread is all the support for USAM. I find it interesting that so many people use it to list and buy, yet their forums are a ghost town.