Private seller, $2000-6000 depending on condition for a M1009, $3000-7500 for a M1008 or M1028 depending on condition.
Drop some for GL prices, if you are willing to deal with EUC delays, SF-97 delays, no test drives, etc.
I have a M1008 that I am going to sell as soon as I get a new starter in it. Ex USMC truck, 58k on it, runs good, has a 12v conversion and a 3 inch lift. Needs tires, paint and a cleanup to be a great truck. I am going to ask $3900 for it with all the manuals on a disk, a gallon of OD paint, and some spare parts thrown in. That will give you an idea to go from.
The axles from a M1008/M1028 are so desired by the folks building custom 4x4's that when you start getting any lower than $2500 it is worth more to part the trcuk out, so if you ever see any runners under that they are a steal.