I just sold my DD 5.1 here on the ProMedia forum (along with my PMU 5.1s - don't worry, I will be upgrading in a few months, just need the extra cash now and won't have the space for a proper setup for a few months anyway). They are around. Mine came in mint condition too (the guy before me had never even peeled the scratch-cover off of the lcd on the DD).
I recommend you setup an eBay notice for when someone creates an auction with "promedia dd" in the title or auction body. It helped me find mine. They can get up to about $300, and honestly, I think they are worth it over the Creative brand DD.
Good luck!