Buying used?

[Enquiries]

I have been looking 2 and 4 bay Synology on ebay for one model back, 918s or 718s. How do you calculate fair pricing for older and in most cases used hardware. For example, I wouldn't pay $600 for a 918+ when I can get a 920+ for $550. But a 918+ with 8GB ram and a couple of 4 TB drives seems like it might be a decent deal.

Thanks
Bob