A few of my snakes were acquired as adults and they are some the sweetest snakes I have. My big Blood and BRB were both a few to several years old when I got them and they have never made an aggressive or defensive move toward me.
My young Bloods made me bleed pretty often when I first got them. I liked getting my big girl how she was more than how I got them.
If you find a adult snake that is feisty and the owner says its just because your new to him walk away. That snake is most likely going to stay that way and may be the real reason they are getting rid of it.