I'm inspired by Alatariel-Silimaure's post and want to jump in with my own recent experience with a lovely seller on eBay.
I bought an MOC pony from her, but when she arrived, I noticed some major discolouration on her back that wasn't mentioned in the listing. After my horrible (and still ongoing) nightmare with barbaras1220, I was afraid to e-mail the seller to let her know.
I finally got up the courage and e-mailed her today. She replied quickly and professionally, offering either a partial or full refund. She apologised for not looking more closely at the pony, saying she just assumed the pony would be in "as new" condition being that she was still in her original packaging. In the end, I decided to keep the pony and get a partial refund. I even offered to send a photo of the discolouration for her peace of mind as I wanted her to refund the money in full confidence. She responded saying she didn't need a photo, that she believed me, and apologised again. It made me wish I could leave positive feedback ten times over since it's so rare to come across sellers who are this polite, patient, and professional.
Yay for more great seller moments!!