Wiring an ammeter with no shunt
Yes, if the voltage that you are measuring is within the 3-30 volt, then you can use the device's power supply to power the voltmeter/ammeter. 

If you scroll down on the ebay link that you provided, it provides a diagram of how to wire the voltmeter. 

If the monitor that you are measuring draws less than 10 amps, then you don't need a shunt.  There should be a label on your monitor telling you its power consumption and from there using Ohm's law you can estimate the amps it should draw and if less than 10 amps then no shunt needed. 

Just ask if it is still confusing. 

Good luck.