In Messenger: How Do I Unblock

No. Facebook does not send a notification to the other person when you unblock them. They will only realize it if they search for your profile and find that it is visible again, or if they try to message you and it goes through.

If you blocked them specifically for Facebook Marketplace, note that there is a separate block list for Marketplace within the settings. Unblocking them on your main profile usually unblocks them for Marketplace, but double-check the "Blocking" settings menu to ensure they aren't listed under "Marketplace bans." how do i unblock in messenger

No. To prevent cyberbullying and harassment "games," Facebook enforces a cooldown period. If you unblock someone, you must wait 48 hours before you can block them again. how do i unblock in messenger