Every vendor making a sale of tangible personal property for the purpose of storage, use or consumption in this state shall collect from the purchaser an amount equal to the percentage on the sale price imposed by the sales tax law in section 144.020 and give the purchaser a receipt therefor. The required amount of the tax collected by the vendor from the purchaser shall be shown separately upon the sales slip or other evidence of sale. If a vendor is a selling agent as defined in section 144.605, and receives compensation by reason of a sale made pursuant to an order given directly to his principal by the purchaser, of which the selling agent had no knowledge at the time of the sale, and in which the selling agent did not actively or inactively participate, the selling agent shall be relieved of all liability for the collection and remittance of the taxes imposed under sections 144.600 to 144.745. Furthermore, if payment is made by the purchaser directly to the principal and the selling agent is unable to collect the tax from the purchaser, the selling agent will be relieved from all liability for the collection of the tax imposed under sections 144.600 to 144.745 from the purchaser. Selling agents may avoid all responsibility for collection of the taxes imposed under sections 144.600 to 144.745, if their principal is a vendor registered with the director of revenue for the collection of such taxes.