It is in no way illegal to make or purchase fake diplomas as long as it is used for the purposes stated by the diploma makers. A fake diploma is no different from any other knock-off item that people purchase every day. You can even purchase a law enforcement badge or ID card from any local toy store which is not illegal either. The legal issues arise from how that item is used.
The companies selling fake diplomas such as www.fakediplome.com are selling them for novelty purposes as stated on their site. In some cases, it may be purchased to replace a real one that has been lost or damaged. In some cases, a person that purchases a fake diploma may, in turn, cause that person to go back to school and get a real one.

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Fake diplomas are also used frequently in productions as props or as gag gifts.
If you are purchasing one with the sole intention of using it to get a job then when it is discovered to be fake you will face the consequences of that deceit.
So if you are looking for a realistic fake diploma, you will not be disappointed with the quality and customer service at www.fakediplome.com