What is the ruling on buying official stamps for an amount exceeding their original price, and are these stamps considered property that may not be sold or bought except at their specified price?

Purchasing stamps is a purchase of a specific service. Whoever owns a service is permitted to benefit from it themselves or to transfer the right to benefit from it to others, either gratuitously or for compensation. This is similar to someone who rents a house; they may sublet it or allow someone to reside in it without charge. Therefore, there is no harm in purchasing stamps for amounts greater than their original price.