Although it is hard to determine if the piece is Jade from the rough sides, it does appear to be Jade on the cut and polished portion. Other than the quality, the type of Jade and where it is from, would determine the value and one of that size could be worth many thousands of dollars. I have no idea as to whether the "N & Hagen" refer to the previous owner or where purchased or location it was found. It is hard to say where the best place would be to sell the piece but I would imagine that for a specimen of it's size, eBay would be just about as good as any! I would recommend having the specimen examined by a professional Gemologist to determine the exact type of Jade, where it is from, the weight in carats, pounds & kilograms and if it is a valuable type of Jade, then get a GIA Certification if possible. All of these will help boost the value of the specimen if an when you decide to sell it.
Frank