Persons engaged in the business of making retail sales at auction of tangible personal property owned by such person or others are retailers, and are, therefore, required to hold sellers' permits and pay tax measured by the gross receipts from such sales.

The amount upon which tax is computed includes he amount charged for merchandise returned by a customer at an auction sale, if the sale is made under an agreement or understanding at the time of sale that the property will not be delivered or that any amount paid will be returned to the bidder.

Sales tax does not apply, however, when an owner of property delivers it to an auctioneer for auction and bids in his own property at the auction.