Will a motor be safe if i over voltage it a little bit?
Just hooking it up to a higher voltage can deliver more current than the motor is rated for. That will reduce the lifespan of the motor and, if the current is high enough, cause it to fail.

However, there are a number of separate issues with DC motors.


Over Current. As I mentioned above, too high a current will overheat the coil and if it gets hot enough, it will burn out or seize the motor. Too much current can also affect the magnetics in the stator.
Over voltage. The windings in the coil have an insulation rating that will fail if the voltage is high enough causing arcing and again, burn out. Further, the brushes will arc more at higher voltages causing heat and faster wear.
Over speed. DC motors are not the best mechanical beasts in the world. Brushes rubbing on the commutator and often poor bearings means over speed will significant reduce the lifespan of the motor.


Having said all that, the number one issue is over-current.

CONTROLLED OVER-DRIVE

Motor rating plates are defined for a static..."Apply a voltage and go" setup. 

If you limit the current to the motor you can in fact drive it at much higher voltages and speeds without significantly over-heating the motor.

This is because one of the governing factors of the current taken by a motor is the speed of the motor itself. The back EMF generated by the motor as it spins reduces the current you can deliver at any given terminal voltage. 

In theory, if say the rated current of the motor is 1A, and your motor drives the load at max speed using less current than that, it is possible to increase the terminal voltage, and hence the speed, until the point where the rated current reaches that 1A level. This must of course be done while limiting the current to that 1A value. 

However, there are limits to this. Obviously you can not exceed the breakdown voltage of the coil, but also you have to remember there are switching inductors inside the motor. These form an impedance that increases with motor speed. As such the max current becomes harder and harder to achieve with speed within a commutation cycle. Also, the motor may mechanically destroy itself at extreme speeds.

EXTRA COOLING 

The other thing you can take into account is cooling. The motor will be specified at a certain ambient free flow air environment. If you can increase the air-flow over the rotor or otherwise extract heat from the core, you can in fact drive the motor harder with little significant effect on the motor other than reduced brush life.

YOUR MOTOR 

Unfortunately E-Bay is a terrible place to buy stuff. Specifications and data sheet links are usually non existent. So knowing exactly what no load current = 3A is supposed to mean is anyone's guess. Plus, what you get in the mail is not always what's shown on the site. However, at &lt;4$ a pop....