Depends on what the uses of the pc. Is it gaming? Or for productivity work, gaming wise I would say cpu don’t degrade as much through normal us, unless previous owner runs it near it’s max oc limit and high load 24/7 through out the day. Consumer markets, the good chip to buy is r5 1600, you can probably get them around 130-150 usd used, also you can try to get the old 6700k or 7700k, you can find them about $200-250 used. Other chips are not recommended as the new i3 8100 and r3 2200g offers great value for around $105-110 new. It’s better to get those cpu mentioned as it uses current Gen ram and motherboards so there is lots of option of getting cheap budget boards. A product line goes beyond its life cycle ( lets say 2600k) for example, new motherboard and ram are selling above msrp, and you are force to buy those component which are far likely to fail. Productivity is different as you can find lots of decommissioned Xeon chip and ram from Ali express. I’m not sure about it’s pricing.
I just bought a second hand CPU 2700x and a second hand motherboard Asus b350 and they were in perfect condition. But you need to communicate with the seller and ask questions such as if it has been overclocked etc . You should buy through eBay as it has protection against items being broken for example if you buy a motherboard and it wasn't described like in the description then you have the option to refund it back to the person . However other sites like gumtree have no protection so you would need to be more careful but then again if you use a site like gumtree you would go to there House and buy it as it is different to eBay . If that is the case you can ask to see the component working in the pc and if they refuse don't buy it. Hope you find this helpful