From the manual:

-e / --exit-status:

Sets the exit status of jq to 0 if the last output values was neither false nor null, 1 if the last output value was either false or null, or 4 if no valid result was ever produced. Normally jq exits with 2 if there was any usage problem or system error, 3 if there was a jq program compile error, or 0 if the jq program ran.

So you can use:

if jq -e . >/dev/null 2>&1 <<<"$json_string"; then
    echo "Parsed JSON successfully and got something other than false/null"
else
    echo "Failed to parse JSON, or got false/null"
fi

In fact, if you don't care about distinguishing between the different types of error, then you can just lose the -e switch. In this case, anything considered to be valid JSON (including false/null) will be parsed successfully by the filter . and the program will terminate successfully, so the if branch will be followed.

Yes, JQ can do all that. Here is how I'd write it:

jq -nr '
def valid_key: test("^[A-Za-z_][0-9A-Za-z_]*$");
def valid_value:
    type != "array" and type != "object" and (
        type != "string" or (test("\u0000") | not)
    );
def valid_input:
    type == "object" and (
        to_entries | all(
            (.key | valid_key) and (.value | valid_value)
        )
    );
input |
    if valid_input and isempty(inputs) then
        to_entries[] | "\(.key)=\(.value | @sh)"
    else
        "bad input\n" | halt_error(1)
    end'

I guess you didn't notice the JavaScript errors in your console.

You have a syntax error...

seller: {required: true, minlength: 4, maxlength 20}

should be...

seller: {required: true, minlength: 4, maxlength: 20}

(You were missing the colon, :, after maxlength)

DEMO: http://jsfiddle.net/9z4G6/

BTW, you can also combine the minlength and maxlength rules into the rangelength rule...

rangelength:  [4,20]

http://jsfiddle.net/9z4G6/1/