I would imagine this is dependent on which state (or country) you live in. I am just curious in terms of the US, could I sell property for myself or for others without obtaining a "license to sell" within my home state?
No you do not need one. However, if you are selling properties owned by other people and collect a commission, you will need a real estate license. Some choose to work without one but you can get in trouble with the law if you are selling for others and collecting fees for your service.
As mentioned, you do not. I think a lot of people are under the impression that you need it. You only need this when you plan to act as a middle-man for someone buying or selling a home. If you are buying and selling your own properties, no need.
To flip houses, no you don't need one but you are shooting yourself in the foot if you don't have one. Going through the education and testing to get your license is what helps big time. A lot of people fail to really generate huge profit because they never bothered to get a license.