I don't see this option for any of the Campaigns i have created.

Can you share purchased content?

Yes

Ok, so right now, it is not possible to select which purchased material is to be shared. It's all-in or nothing. IN the future, this will be possible.

No i think you misunderstand,  i have created a Campaign and purchased the core books BUT i am not the GM for one of the Campaigns i have created.  I thought there was a option to change who has the GM Role in a campaign you have created.

Ok, so now apparently right now is not possible to "change" role. This is what you can do:

have your gamer create the campaign, then when you join , you allow the sharing. this way your fellow gamer can use your purchased content.

I didn't know we could do it that way. I thought only the DM could share. Nifty!