Read here: https://en.wikipedia.org/wiki/Local_post
More importantly, the references for the entry will give more detail.
Then you choose whether to consider them stamps or Cinderellas (labels). What ever your choice, folks will agree with you and others will disagree with you. But "collect them if you like them" is the way to go.
If you limit "real stamps" to those issues recognized by the Universal Postal Union (UPU) for use on mail from one area to any other UPU country, you will still get arguments. Add valid within the area of issue with issue by a recognized government entity and the argument gets longer.