Electronics are known for poor value retention. However, after a while, some electronics’ resale value will stop tumbling for some odd reason. For example: You might find an early 80s or 90s computer selling for far more money than a 5-10 year-old computer. Why?
In the case of the 80s computer: It might be because it’s a historic relic and people want to collect them for nostalgic reasons. However, there are other reasons consistent with the ones below.
The 90s and early 2000s computers (models prior to 2007): Older point-of-sale (POS) systems may be running custom software built for Windows XP or even MS-DOS. If they can find a replacement IDE (PATA) hard drive or a Pentium processor for that old computer and get their store going smoothly again for $50-$100, it might be worth it.
Spending thousands of dollars to have the software rewritten for Windows 10 — That may not be on the table just yet. Look for your model computer (or parts for it) on Ebay to see how much they sell for, then look into testing/preparing to sell them.