How are you defining value?

I think standard economic thought defines value in terms of preferences.

Such that the value that something has to me is the supremum of how much I value any of the things I would give up in order to have it.

So, if I am willing to purchase an mp3 for $1, then I value it at at least $1. If I would also be willing to purchase it for $2, then I value it at at least $2.

If I value it at $2, and it is for sale at $1, then I would gain $1 of value in that transaction.

If the seller is willing to sell it at any price over $0.50 , then that is how much they value (not) selling it.

So then, in this transaction I would gain $1 in value, and they would gain $0.50 in value.

I think this is the standard way to think about it.