I work as a volunteer for Cancer Research U.K. We have been donated massive amounts of vintage Lego.
Could you please advise how to sell.
BrickLink or eBay. You could get significantly more if you go through the lot and separate out full sets then sell them on BrickLink. But if you don't have the time at least take a lot of really good pictures - in this case an eBay auction would be the better choice.
Brick Link is a site for buying and selling Lego. Although it focuses on Lego fans, who will take the time to enter each piece separately, they do have the option to sell 'unsorted lots', so perhaps that's an option for you.