Duffer then rejected Slicker's bike on the ground that he had already bought Koolcat's"What is the probable legal effect of Koolcat's conversation with Slicker and report that he (Koolcat) had sold his Sujocki to Duffer on November 10?
Unless a contract had already been formed between Slicker and Duffer, Koolcat's report to Slicker operated to terminate Slicker's power of accepting Duffer's offer. 
This report has no legal effect because the offer had been made by a prospective buyer (Duffer) rather than a prospective seller.
Koolcat's conversation with Slicker on November 11 terminated Duffer's original offer and operated as an offer by Koolcat to buy Slicker's Sujocki for $950.
This report had no legal effect because Duffer's offer was irrevocable until November 12.