It`s all down to what you are paying, the specification of the laptop and what you expect.
If buying from a `second user` retailer then you need to look at the new cost of the specification of what is on offer and what is being asked for the warranty on the laptop you are looking at. But you shouldn`t pay more than 50% of new price with a 6 month warranty for a very good condition unit. If buying from, say Morgan Computers, they offer good warranty, support and refund/exchange facilities.
If buying privately, you may get the unit `as seen` - this means little if from an internet source. If so the price should be much lower as there is no warranty and the condition may not be as good as you would expect.
Make certain the retailer is reputable; better to buy a legitimate product with a legal OS. This applies to a private sale or a retailer.
You may also try to find what the reason for any replacement parts was. Has it been dropped (look for damage), has it been `well used` (the case/pad/hinges may show signs of this), has it got a new battery (depending on age) and is it complete (charger, disks, etc.)
Given the price of new lower end units you may find it better value to consider one of these as they are likely to be better than buying second hand privately on the internet.