Are you buying or selling? If your selling you know what you paid for it and what kind of condition it's in, so go from there. If your selling to buy another, how bad do you want to sell? are you selling to a Friend, Relative or a unknown? A Friend or Relative uusually gets a better deal, a unknown will be even tougher as they know you want to sell and will try for the best price possible.
Anytime you sell, you should never expect to get what you want, you will lose and you should consider this as if you lose too much it may be worth while to keep it and add it to your collection.
If your selling to up-grade and need the cash, you usually will get 1/3 to 1/2 of what you paid and that's it.
A used ML is only worth as much as someone is willing to pay for it.
Good Luck to you!
(BP)