You´re probably on eBay.com so excuse me if the help´s not accurate due to me using eBay.de
1) My eBay -> Selling -> Activ = there you find your active listings which can be edited there.
2) The seller should/has to cancel it, you shouldn´t be worried about that.
3) I honestly have no clue, afaik that´s not even possible but idk.
4) When he has such a policy and also at the point where you bought/paid the item he has to accept the return. If he refuses to do so you can safely contact eBay and explain your issue with said seller. You might get your money back that way!