A conditional sale is exactly what it says, the sale of your property is conditional on the sale of the Buyer's property, which may or may not be under contract at the time of your Offer. Often times, it may not even be listed for sale yet or they may be attempting to sell by owner. Any of these conditions can have an impact on what may result with your sale.
This is why if you agree to a conditional sale, you should set forth the terms. You need to make sure the home is under contract or not, if it is listed in the MLS and what are the terms the Buyer will agree to for anyone who purchases their property. If your Buyer's property is not under contract yet, you may to specify they do not accept a contract that is also a conditional sale of yet another home. There are a number of things to think about other than just yes to a Buyer that is conditional on the sale of their home.
You will find this in section 10.7 of the Colorado Approved Contract to Buyer and Sell Real Estate. If the Buyer notifies you by the date in the contract that they were unable to get their property sold, they have the right to terminate the contract with a return of the earnest money. If it goes past this date and they were unable to get their home sold, they would be forfeiting the earnest money.