To sell your ONE you need to have a Metamask account and deposit ETH into it, so sell your ONE for ETH and insert your Metamask address and then sell ETH for EUR or USD on Metamask.

More info :

This Bridge between Harmony ONE and Ethereum is needed because of the lack of decentralised off-ramp services for Harmony ONE so Sparkle found this decentralised solution implementing the most used decentralised ETH wallet and making you swap between ONE and ETH.