effect, if any, of input voltage on RPM of DC motor
But does input voltage have an effect on RPM?


Yes. A DC motor speed will be proportional to voltage (for a given load resistance).


  I'm wondering why the seller provided a 12V adapter when the motor says 24V. Is the seller maybe relying upon the 12V DC adapter to reduce the RPM? 


Possibly to reduce the RPM but more likely to limit the current should the grinder stall. The motor is rated at 6 W. Power, voltage and current are related by \$ P = VI \$ (where I is current). At 24 V and 6 W we can calculate maximum continuous current, \$ I = \frac {P}{V} = \frac {6}{24} = 0.25~A\$. 

Current will increase with increasing load reaching a maximum at stall. You could estimate what the stall current will be by measuring the motor coil resistance and calculating the stall current from Ohm's Law, \$ V = IR \$, as follows: \$ I_{STALL} = \frac {V}{R} = \frac {24}{R} \$. I suspect that you'll find that it's more than 0.25 A.

By reducing the voltage to 12 V the stall current will be reduced by half but since both V and I have been halved the power dissipated during a stall will be one quarter of the 24 V value. \$ I_{STALL} = \frac {V}{R} = \frac {12}{R} \$ in this case.


  Should I be searching for a 12V DC (as supplied by the seller) or a 24V DC adapter?


Does it turn the coffee? Try it out with a 12 V, 0.5 or 1 A, wall-wart PSU and see if you can make some measurements. Report back!