My Life Sucks, Can I Get A Discount / Free
This isn’t really a scam, but after making a purchase, they will ask for a discount, and use a sob story to try and get it.
The fix: tell them they can cancel the sale. Allow the cancellation, and then block the buyer.
Item Not As Described
This is not really a “scam”, but a problem resulting from your incomplete listings. Some buyers are looking for flaws in listings, and will purchase the item, then point out the flaw, and request a refund or a partial refund.
The fix: you just need to be accurate in your listings, and list all flaws. Photograph the flaws and point them out. If they catch your error, then offer a partial refund. Otherwise, apologize and take a return and give a full refund, including refunding the postage price.