I have a few times, and I always want to, but I don't anymore unless we're still emailing each other and then I thank them for sure. I decided not to otherwise because I don't want them to think I'm bothering them and I don't want them to feel like they have to respond to my thank you. I got a generic "thank you for leaving feedback" coupon once or twice, but I've never had a seller send me a personal thank you email for leaving feedback.