That same eBay seller offers this 10W LED-driver power supply
.To expand a bit on the voltage & current issue...
LEDs (all diodes actually) are non-linear. A voltage change of 1/10th of a volt might change the current flow by a factor of 2. On top of that, the forward voltage drop (at a constant current) varies with temperature, and from part-to-part. So the proper solution is to use "constant current" power supply. A constant-current power supply tries
to supply the same current no matter what load is attached. (At some point if the load resistance is high-enough, the power supply cannot supply enough voltage, and the current will be lower. But, within it's normal operating conditions, the current is (approximately) constant.)
With "normal" low-power LEDs, a constant-voltage
power supply and a current limiting resistor can approximate a constant-current source, and thats how it's done with "regular" LEDs. The higher the supply voltage, and the higher the voltage-drop across the resistor, the better it approximates a constant-current source. But, the resistor usually wastes more power than the LED is consuming, so a current limiting resistor is not efficient and not practical for higher-power LEDs. A constant-cuirrent switching
supply (which uses an inductor) can be nearly 100% efficient. Just about anythhng you buy that has 1W or higher LEDs is going to have a constant-current switching supply.
The downside to a constant-current (switching) supply is complexity and cost.
A "normal" power supply is constant voltage
... A good well-regulated 12 power supply will supply a (approximately) constant 12V as long as the input voltage and current-load are within spec. Actually, most "things" are constant voltage... For example the Arduino's output pins are (about) 5V (when "on" ), as long as you don't exceed the 40mA output-current rating.