If they are RT600i, then probably worth just a little more than the bare 600s. You can tell either from serial number, or the "i" series has blue woofers. Also the "i" series has double grills covering the entire front of the speaker. My fronts are 800i's but my 600s that I paid $80 for are the older model.
ok yeah they are the 600i's .. so the only difference is the grills? the guy who i'm buying them from doesn't know anything about them or audio in general .. he just has money and wants things .. He is also getting rid of his Denon AVR-4306 .. which i may snag from him also if the price is right .. doesn't have HDMI in and out .. but its still a decent receiver