it already exists at https://haveno-reto.com. You do need 0.11 for the security deposit, but that can be gotten with either XMRBazaar or the get your first monero rooms on SimpleX or Matrix 

 Thanks, didn't knew Haveno was already working. Will test it. 

 Also on this topic, woudln't this kind of transaction with an arbitrator already be possible with the nostr NIPs? 

 Yep, it's been running for just over three months already and has like 650 transactions. 

 I don't know nostr well enough to say whether it would be possible or not.