The value of something is how much you can get in return for the thing from a purchaser that you can find. Even "value is what we agree it is" isn't really right, it's what you can concretely get for the thing. (For instance, the fact that a certain Magic the Gathering card is listed in some book for $20 is meaningless if you can't find anyone who will give you more than $5.) There is no other definition of value that's actually useful, but there are plenty of others out there that will lead you astray.
The value of something is how much you can get in return for the thing from a purchaser that you can find. Even "value is what we agree it is" isn't really right, it's what you can concretely get for the thing. (For instance, the fact that a certain Magic the Gathering card is listed in some book for $20 is meaningless if you can't find anyone who will give you more than $5.) There is no other definition of value that's actually useful, but there are plenty of others out there that will lead you astray.