County treasurers are allowed to hold tax lien auctions between September 1 and the second Monday in December. Most counties hold their sales in late October or November. The premium bidding method is used for most auctions and the investor only earns interest on the minimum bid and not the "premium". The "premium" amount is the difference between the winning bid and the minimum bid. There are a few smaller counties in Colorado that use the rotational bid process. In rotational bidding, each investor that is registered for the sale will receive a bidder number that will control the order in which they bid during the auction. The tax lien certificate amount and the interest rate remain constant. The auctioneer will begin with the first investor and ask if they are interested in investing in the first tax lien certificate. If they want it, they will become the successful bidder and then the auction moves to the next property. If the first investor didn't want the property, it is offered to the second bidder, third bidder, and so on, until each property is either sold, or offered to everybody in the room and there are no takers.
Colorado County Reference