This is what happens when the language is unclear - the word "value" has different meaning in each context.

"Value" as in "utility", and "value" as in "currency" or unit of account.

When trading some currency for utility, each side of the transaction gains something more than what they gave up. The buyer of food got food they needed to eat. The seller of food (presumably has so much food they cannot eat it all) gains extra currency that they can use elsewhere.