Hi Tim,
You can always look for comparable pieces on eBay. That is a pretty good indicator of what willow is going for now. You can now post on our site willow for sale, so once you establish a price might wish to do that. Using price guides in books might be helpful initially, although the guides usually outdate themselves pretty quick. If you can post a photo of the willow and mark, someone may be able to help you identify the maker and then a rough idea of value. Hope this helps some.