question about guest dm'ing

if i buy one of the official wizards map packs for a campaign from the marketplace, can i promote a guest to be the dm and will they be able to access the maps i paid for? i prefer to be a player but would also like to have access to the maps just incase i ever wanted to dm. 

A co-gm only has access to the creator's assets that have been added to the game.  Put "in-play", if you will.  A co-gm will not have access to the creator's assets. So basically, the creator will need to drag maps and tokens to the page(s) and then the co-gm can manipulate them.  A co-gm can also use their own assets in the game as well.

If you're talking about a Module, you can create the game and promote them to Co-GM and they will have access to the full contents of the Module in that game. If you're talking about an Addon, you can add the Addon to a game you created and promote them to Co-GM and they will have access to the full contents of the Addon in that game. If you're talking about a Marketplace Set, you can drag each item in the Set into a game you created and promote them to Co-GM and they will have access to the copies you've placed in the game. (As Vince notes above)

This question seems to come up often and the answer always seems to be "it depends...". lol  Aaron's answer is " Wiki-Worthy ".