Post the datasheet for your LEDs. Cheap LEDs on E-Bay do not have a detailed datasheet.
LEDs do not have a certain voltage like a simple light bulb. LEDs have a minimum, typical and maximum forward voltage because they cannot make them all the same like simple light bulbs.
If the actual forward voltage is lower than you think then without having a resistor to limit the current then the LEDs will burn out.
If the actual forward voltage is higher than you think then without having a resistor to equalize the voltage then the LEDs might be very dim or not light.