To sell resources to Antonio, you must have the required amount of resources in your backpack's salvage area. You can transfer resources from your stock by first having your backpack's salvage area open (by clicking the little hammer icon), and then clicking the resource inside your stock.
Antonio will only buy one type of resource a day which is chosen by the server (resets 4am UTC).
The exchange is repeatable with the same resources, each time the price will reduce (e.g. the first time you sell to him for 20 , the next one for 18, then 16, 14, 12, 10, 7, 5, 3, 1...). Once you reach a price of 1 , it stays that way for the rest of the day.
He buys resources at the following rate: