How to Prove That the Seller Received Payment

My bank deposited the money into the seller’s bank days ago, but the seller is claiming they didn’t receive it. Other than screenshots of the transaction confirmation (already sent to the mediators), how can it be proven? Should the mediators call the bank and ask?

your proof is the record from your home banking, the seller should give proof of the funds not being there, like, recording a video of himself opening his personal account details, and show that no incoming transfer was received.
You can obviously ask your bank to provide the proof from their side that the transfer was successful