Can a message sent by me in the inbox deleted?
Do you mean can you delete a reply to a buyer so it’s not visible to you, the buyer or Fiverr CS?
Nope, once it’s sent, it’s sent.
And if I delete the whole conversation, can the buyer see it?
Yes they can.
A message/conversation can’t be deleted from the buyer’s end by deleting it from your side.
This was of great help indeed. Thanks alot.