Each MRE is labeled:

U.S. Government Property
Commercial Resale is Unlawful

However, there are no specific laws that do not let you resale or buy military MREs.

The Defense Department has been unable to show them any regulations or laws specifically outlawing the practice.

That said, MREs can be purchased by civilians directly from the contractors who supply MREs to the U.S. Government. These MREs are very similar to genuine US Government MREs, differing only in minor details (i.e. design of case and bag or type of spoon).