Used equipment rarely sells for more than 50% of new value. Depreciated value is usually decreasing at a rate of 20% of remaining value annually. Auction values can be as little as 1 or 2 %, but usually 10 to 40% of new value, while re-sellers (especially if reconditioned or warranteed) will try for 35 to 65% depending on conditions.
Some types of equipment, furniture and equipment are virtually value-less and worn out or used up at about five years of normal commercial duty. Often a closing ship had less than normal traffic and therefore minimal usage/wear on the capital goods.
The POS system is likely ok, but electronics do not hold value well, since keyboards, printers and pc's wear out and out-date quickly. The 8' display loses more value if it is refrigerated because the compressors need replacement, so consider the price PLUS a compressor replacement at this age when determining value. Mahlkonig grinders are some of the most durable machines and can be refurbished. I sold some heavily used Guatemala and VTA6 models that are still in daily use in their new homes - some still have original motors at 15 to 30 years of service. Switches and discs are worth replacing, but discs are not cheapos.
Be prepared with a price you are willing to pay (and reasons to support your price as best value), and if the seller is asking too much, present your offer with explanation, then the seller may see the light and accept your offer.