My guess is that the eBay drivers are designed to output a forward voltage of 3 volts or so, which is the typical forward voltage of a single LED. But that's just a guess. These appear to be constant current drivers, so output voltage is really not much of a concern. You feed the driver 12 volts DC and it produces 700 mA of current. So provided the LEDs you use can handle 700 mA, you're good to go. There's a limit to how many LEDs that little driver can run though, but in your case, if you're only running one off each driver, should be no problem. Hope that makes sense.
Edit: as far as parallel vs series set ups, you're really not restricted to the number of LEDs either in parallel or series. It's the power supply that limits the number of LEDs you can hook up in any given configuration. If you want keep things really simple all you need is a big enough 12 VDC power supply and you can keep adding MR16 12 volt LED garden lights until the cows come home. The MR16 LED bulbs already have the driver built in to run at 12 volts. MR16s are available with LEDs from one to four. They're also available on eBay for a very good price, as are the sockets they plug in to.
This is a 4 watt MR16 bulb that runs on 12 volts DC. If you have a 100 watt 12 VDC power supply, you can run 10 of these safely, in parallel.
