HDD have to be bought new, as well as anything mechanical (eg fans). But for motherboards, CPU, RAM and SSD, there is great value in buying used enterprise hardware on ebay. It is generally durable hardware that spent a quiet life in a temperature controlled datacentre, server motherboards from 5 years ago are absolute aircraft carriers in term of PCIe lanes and functionalities. Used enterprise SSDs are probably more durable than a new retail SSD, plus power loss protection and better performance.
The only downside is slightly higher power consumption. But just bought a 32 core 3rd gen Xeon CPU + motherboard, 128GB RAM, it idles at 75w without disks which isn't terrible. And you can build a more powerful NAS for a third of the price of a high end Synology. Unlikely that the additional 20-30w idle power consumption will cost you more than that.
The only downside is slightly higher power consumption. But just bought a 32 core 3rd gen Xeon CPU + motherboard, 128GB RAM, it idles at 75w without disks which isn't terrible. And you can build a more powerful NAS for a third of the price of a high end Synology. Unlikely that the additional 20-30w idle power consumption will cost you more than that.