A m-of-n transaction means there are a total of n parties in the transaction, each with their own private key.
The transaction need to be signed with any m parties, to make it a valid transaction.
So, with a 2-of-3 setting, you can get 3 persons to be escrows, and you will have no problem even if one of them disappears.
For 2 - 3 you would have the buyer and seller each have one "key" and the escrow person would have one key
Ideally the transaction would go smoothly and you both sign a TX that sends the coins to the seller. If there is a dispute the escrow person will review the facts and send the coins to the buyer/seller based on the facts.