I found an interesting way to trade securely, with a few ideas of my own.
1. Build a room/building that is split in two by a glass wall.
2. Build two 1x3 canals under the glass wall, water flowing from side A to side B, and side B to side A.
3. The person merely needs to throw the items into the canal, where they will float over to the other side.
4. There is ONE entrance to EACH side of this room/building with an iron door and an internal switch (on/off, not a button), to keep other people from interfering with a trade.
Example:
Person A wants to buy cobblestone from person B. Both step into the "trading room" via doors 1 and 2. When inside, both lock the doors so no one else can enter. Person A uses the money command to pay person B, where person B then throws the cobblestone into the canal, where it floats over to person A. Afterward, both unlock the doors and leave.
I suppose this method could also be used as an alternative to a public chest, especially in a shop or bank situation. The iron door and "locking yourself in" is a key importance. I figure all banks should take this into account when designing banks so others do not come up and pop the chest mid-trade.