I'd most likely pass on the Buckthorn, but try burning some and see what the smoke smell like.
The Walnut, if the tree is big enough, can be worth some money. If you use it to smoke with, I would mix it in small quantities with oak or pecan. It is a very strong almost harsh flavor and too much will give the food a bitter taste.
I have tried it and didn't care for it. JMHO