You have the 7¢ McKinley from the years 1923-30. It is a common stamp and is worth no more than a few dollars whether used or unused. Taking the easily identifiable varieties first;

The stamp has the letters Kans. on it

The value of this stamp (#665) can be found by clicking the numbers 658-668 on the menu to the left.

The stamp has the letters Nebr. on it

The value of this stamp (#676) can be found by clicking the numbers 669-679 on the menu to the left.

The stamp has perforations on three or more sides
and you count ten perforations along the bottom

The value of this stamp (#588) can be found by clicking the numbers 575-596 on the menu to the left.

The stamp has perforations on three or more sides
and you count eleven perforations along the bottom

To determine the difference in the perforations between #559 and #639 you will need a copy of the very common 1932 3¢ Washington (#720). A picture of which is shown below. Follow the instructions below

The value of #559 can be found by clicking the numbers 551-561 on the menu to the left.
The value of #639 can be found by clicking the numbers 632-642 on the menu to the left.