You have the 7c Stanton from the years 1871-1875. It is fairly uncommon. To value the stamp you have to first look for a what is called a 'Secret Mark' and look for an embossed pattern, called a grill (an example is shown below), on the reverse of the stamp.

If the stamp has a grill embossed into the stamp, then click here.

If it does not have a grill and does not have the secret mark (a semi-circle shown below) then click here.

If it does not have a grill and does have the secret mark (a semi-circle shown below) then click here.

Showing semi-circle secret mark

Without semi-circle secret mark

Example of a grill

This stamp was reprinted in 1875 and it can be viewed by clicking on the numbers 167-177. These are all accounted for, so you will not own this.