sell or buy wholesale, store
躉 centers on dealing in large quantities: buying or selling goods wholesale, and by extension storing bulk supplies.
躉 combines 萬 (ten thousand, a large number) with 足 (foot, but here likely used as a semantic component related to goods or movement), suggesting the idea of handling goods in large quantities. The exact historical development is uncertain.
Picture a warehouse where ten thousand (萬) items are moved on foot (足) in bulk: that's wholesale buying and storing.
For トン, imagine a ton of goods being stored wholesale: ton -> トン, and the warehouse is full of bulk items.