What is the value of an year 1840 piano?

The best thing to do is to go to someone who specializes in selling pianos...they'll be able to tell you how much you could get for it. If it's in moderatley good condition, you could probably get alot of money for it from someone who collects old pianos.

It is best to get the serial number which can typically be found - http://www.lindebladpiano.com/piano_Serial_Number.html and also email photos of your piano to a piano company that specializes in vintage pianos such as -

http://www.lindebladpiano.com/

http://www.countrypiano.com/

http://www.vintagepianoshop.com/ Also my opinion is i would pay about 2000 bucks