1. Condition: I've had three year old units here that weren't worth the cost to remove them from the truck, and 10 year old machines that could be put on my showroom floor with a wipe down with a damp cloth and replacment of a few fittings.
2. Type of machine: Air cooled engines have shorter lives than liquid cooled engines.
3. Brand/Model: Is the company still in business? If so, is the specific model still made and supported by the manufacturer?
4. Support: Is this a machine that you can get fixed (relatively) locally? Does the closest/most reliable service center work on that model, and even if they say that they do, are they familiar with it?
If you are selling, not buying, is there time pressure on you to sell it? There are times of year in different regions that cleaners are more inclined to make large purchases than others. What assurances can you give the buyer that your unit is in good working order? Is there a local dealer who can check it out and give an unbiased condition report?
All of these things will help you establish value for buying or selling a used machine.