The datasheet shows that it's a class .5 instrument; that's a 1/2% accuracy according the the JIS standards.
This is an electrodynamic wattmeter:
Wattmeter - Wikipedia, the free encyclopedia. The current coils and the voltage coil multiplier resistors dissipate a little power themselves. The small VA ratings you see are the values of power consumed by the instrument itself; that's what the data sheet refers to as "Low self-consuming power". You need to take these into account for extremely accurate measurements.
The meter shown on eBay has voltage ranges of 120/240 volts, and current ranges of 5/25 amps. This means that the instrument can have 4 different full-scale ranges--600 watts, 1200 watts, 3000 watts and 6000 watts.
There is a single scale reading from zero to 120. The indicated number has to be multiplied by an appropriate scale factor to give the actual power reading. The scale factors are shown on a label on the front of the meter. There is a picture of this label on the eBay site for the meter.
Since the meter responds to DC, you can calibrate it yourself. You need a voltmeter that can measure DC voltage accurately, and an ammeter that can measure DC current accurately; you could use 2 DVMs for this. Since the voltage and current coils are electrically isolated, you use two different power supplies and apply accurately measured voltage and current to the meter. Multiply the value of the DC voltage and the DC current and you have the DC power in watts, which the meter should indicate.
You can see from the data sheet that these meters are available with much smaller current ranges, which you might need to use if you want to accurately measure low standby power consumption.
These meters are expensive if you buy them new; around US$600 each. The one on eBay is a good bargain.