You should always quote the f number of a lens for such a question. As it happens, there are both consumer and pro Sigma lenses with that same range.
Once you have that information, you can do advanced search in ebay.co.uk for only the completed items to see the going price there.
The resale value of third party lenses drops like a rock.
Thanks. I will ask the f number for the Sigma but I'm assuming the Nikon lense is the standard kit lens. Is it still a good price though? Thanks