When selling a business, common mistakes include taking too long to prepare for the sale, informing customers and employees too early, and underestimating the length of the sale process. The document recommends beginning preparation when drafting a business plan by maintaining detailed records and growth plans. It also advises not informing staff and customers until the sale is finalized to avoid losing key members and customers. Lastly, it notes that fully running the business until the sale is finished prevents issues that can hurt the sale and bottom line.