RS-232 Buffer circuit
If I understand correct, here is your circuit: 

RS232 in -->TTL out-->[Sniffer circuit]TTL in-->Rs232 out

in other words: 

[device1] &lt;--> MAX232 &lt;--> [sniffer circruit] &lt;--> MAX232 &lt;--> [device2]

That looks good to me. The fact that each side of the circuit is talking directly with a MAX232 should give you plenty of "isolation". You may need to cross the lines (or just use a null modem connection on one side) if you're talking CPE-CPE or computer to computer. 

Other than that I don't see any issues.