Cereal boxes have been collectible for awhile. Obviously,depending on who or what event (sports celebrities) is depicted on the box will be part of its determined value. It is recommended that you take the cereal out of the box because it can get moldy and/or bugs in it which will eventually eat up the box too! Usually you open the box at the bottom, take out the bag of cereal and reglue shut the bottom. Box is ready to display or can be folded for storage. Like any paper collectible issues arise with condition, displaying and storing. I have a sense there is a limited market out there for cereal boxes. I believe that their value and prices like in many collectible areas are down.
While I am not 100% positive, I believe that you are able to (since you would, most likely, be selling to a collector/dealer. And, they would never even THINK about opening the boxes…let alone eating the contents. lol). I am VERY interested in finding out the answer to this, also. My late brother-in-law had a MASSIVE collection of old Wheaties cereal boxes…and, I have a sinking feeling that my in-laws threw them out when he passed away. 🙁
Thanks for asking about this. Have a great day.