On the other buyers guides, I spent quite a few years compiling a list of sites that would hopefully cover most currently listed cars of any given model. Panoz has the disadvantage of not even being included on some major sites like AutoTrader and AOL Autos. Here it is in the order I see most potential to find Panoz Esperantes:
The largest and best Panoz forum. A great place to find enthusiast-owned cars and some you may not find listed anywhere else.
You can find everything on Ebay including Esperantes. It doesn't always have listings, but you may get lucky and find one without a reserve.
The only major classifieds site that specifically includes Panoz in their options. There's often 5-10 Esperantes listed at any given time.
Craigslist - Google Site Search - Bing Site Search
I'm not a fan of some of the Craigslist aggregator sites and Craigslist itself shuts down the good ones anyway. I've always just used Google and Bing site searches to find Craigslist listings. It's often the best way to find great deals.
These sites’ listings are generally covered by one of the above sites, but not always.
Claz used to be CraigLook which was awesome, but Craigslist shut down that part of the site. It does do a good job of putting together some miscellaneous local classified sites still.
One of the largest classifieds sites out there.
Another site aggregator. Most of the the time it has listings you can find elsewhere, but every once in a while it has a new one.
A slightly less accurate listing aggregator, but has new stuff some times.
Ebay Classifieds never really took off like I thought it would, and there's not much on it most of the time. Some one could list an Esperante on it sometime though.