Not all JC-120's are created the same. Just like with Fender amps, some models are more rare and/or more desirable than others. I just thought I'd reinforce that point. To make a general statement like JC-120's aren't worth that much, and then pull up prices on other JC-120's is misleading, because there are multiple models of them that are of greater or lesser worth to people. And the one the OP is talking about is the one that is typically of the greatest worth.
To me that sounds like a reasonable deal for an original JC-120 with the black "Jazz CHorus-120" text, if it's really clean.