Legal analysis: the purchase contract can be stamped with the official seal or the special seal for the contract. The official seal refers to the seal used by the unit, which proves that the unit is engaged in production and business activities abroad. Stamping the special seal for the contract on the contract indicates that the parties have completed the offer and acceptance stage of concluding the contract, and finally confirmed the rights and obligations of both parties. It is also confirmed that the contract was established by both parties through consultation and has legal effect on both parties.

Legal basis: Article 490 of the Civil Code of People's Republic of China (PRC), when concluding a contract, if the parties conclude the contract in the form of a contract, the contract is established when the parties sign, seal or press their fingerprints. Before signing, sealing or fingerprinting, one party has fulfilled its main obligations, and the contract is established when the other party accepts it. A contract shall be concluded in written form as stipulated by laws, administrative regulations or agreed by the parties. If the parties do not do this in writing, but one party has fulfilled its main obligations and the other party accepts it, the contract is established.