CS Buy and Sell is not Ebay, the seller doesn't have to adhere to any rules or auction or who bid first gets first. It is a normal buyer/seller market and the seller can choose to sell to anyone he wants. Similarly, the buyer can bid and later withdraw if he wants as well. No consideration is provided and no contract is binding unless both sides explicitly agree to.
To try to make CS a bidding auction platform like Ebay is not the intention anyway.