Article 38 of the Measures for the Administration of Invoices of the People's Republic of China clearly stipulates: those who print, forge, alter, buy or resell invoices without permission, and make invoices' producer seals and invoices' special anti-counterfeiting products without permission, shall be sealed up, detained or destroyed by the tax authorities according to law, and their illegal gains and tools for committing crimes shall be confiscated, and a fine of not less than 10,000 yuan but not more than 50,000 yuan may be imposed; If a crime is constituted, criminal responsibility shall be investigated according to law.

The purchase invoice belongs to the above-mentioned "buying and selling invoices". Whether it is illegal or not, you can decide for yourself!