Hey Bob,
I'm in the market for a used MXT myself and have spent time looking at the DFX too. Use "Whites DFX" on eBay to see what is currently for sale and also from that same page on the left look for "Completed Listings" and follow that link, give you an idea of the current market...
I've seen DFX's from $600 - $900, on eBay or other forums. On eBay check the sellers "See Detailed Feedback" to get a feel for the seller...
Me, if I had my choice I'd buy used from a local dealer 1st, he'll know the machine... There are some MD dealers on eBay, one stands out in my mind, I read his auctions and you know it's a great machine and that he knows his stuff. Buying from a authorized whites dealer (online or otherwise) would be my 1st choice.
You prolly already know this but the 1st digit of the serial number is the year it was made. I've had great results asking eBayer's for the serial numbers.
All that aside, I'd still rather get a used one from a local dealer, even 4 hours away, gives me alot of security on a $600 - $900 purchase. Best one I saw on eBay was just under $700.
Good luck in your search.
Scott