Nothing, there is no penalty to either collector if a trade offer is responded to and declined. If your trade is declined, make a trade more appealing by including more high-quality cards in your offer. Also, check your trading partner's wishlist to be sure you are offering cards that are of interest to that collector. Finally, you can directly message your trade partner to see if they might have any special requests to include from your collection.
After 48 hours, trades that are not accepted or declined expire automatically. There is no penalty to the trader if an offer expires with no response from the trade recipient. But, allowing a trade to expire without a response may impact the trade recipient’s trader grade. Make sure to sign in every day to respond to trade requests and keep your trader grade at an A+!