Thank you for all the advice everyone, I really appreciate it!
I contacted the seller again and she is still refusing to issue a refund, so I filed a PayPal claim. In addition to the stain, the bag has minor marks and scratches on parts that the seller didn't show or mention in her listing. She claims that they are due to shipping (obviously not, since the bag was in its dustbag) and blames me for not requesting insurance - are you kidding me?! It is her responsibility to list and show all imperfections, I shouldn't have to purchase insurance and give her an easy way out so that she can blame shipping. And I agree, if it is so easy to return the bag then I don't get why she would go through the trouble of listing/selling it instead of returning it herself. Ugh, it's just frustrating. I'm crossing my fingers with the PayPal claim after hearing about all the horror stories. I will update you guys on how it goes.