City officers shall not be interested in any contract made by them in their official capacity, or by any body or board of which they are members. Any wilful violation of the provisions of this section shall be a ground for removal from office, and shall be deemed a misdemeanor and punished as such. City officers shall not be purchasers at any sale nor vendors at any purchase made by them in their official capacity. Any contract made in violation of any of the provisions of this section may be voided at the instance of any party except the officer interested therein.