You would have to tell us the model of the laptop. There are a few key factors IMO that determine a laptop's used price:
1. Generation of processor: The older the generation of the processor, the cheaper the laptop. Sometimes laptop manufacturers do put older processors into certain laptop lines.
2. Number of cores: A lot of consumer or business laptops only have 2 cores, those that have 4 usually can demand a much better price.
3. Processor designation (i3, i5, i7)
I7 laptops l usually demand quite a high premium.
4. Business laptop: Business laptops usually hit the used market in waves after their warranty ends, I think for business laptops it is 3 years. The price usually drops quite dramatically at 3, but 4-5 years is usually when you can get some great deals.
5. Screen:
Some laptops of the same model do feature different screens at different resolutions. Be on the look out if you think a price is too high or low, this could explain why.