For a physical barrier:
* Enclosure
* Barrier
* Hedge
* Wall
* Railing
* Palings
* Picket fence
* Gate
For the act of buying and selling stolen goods:
* Receive
* Deal in
* Dispose of
* Handle
* Traffic
For the act of sitting on the fence:
* Hesitate
* Indecisive
* Uncommitted
* Neutral
For the act of fencing in sports:
* Parry
* Deflect
* Block
* Guard
To give you the best synonym, I need more context. What is the sentence you're trying to use it in?