The "Blue Book" and like publications are no more than structured depreciation schedules. They do not appear to consider desireability of the equipment and do not adequately
address the overall condition of the equipment. As most used equipment managers know, some units are almost worthless even in good condition (for various reasons) and others hold their values extremely well for a very long time.
Therefore, your options are somewhat limited. Either call and get other trusted opinions or have a lot of experience.