As with any second hand system you can only get what the buyers are prepared to pay.
With low priced offers and warantee ( Aldi is good systems with 3 years ) The second hand market is virtually ststic.
You will get a better value looking at on line local free Adds.
Wilst selling as parts you need to check what those parts sell for new.Chronus nensioned your O/S which if it is OEM ( already installed on purchase ) you cannot sell it or transfer it to another System
As a base unit I shoul not think more than £50 with a monitor it will depend on type and size