I can understand your disappointment. You say you weren't aware of it until you were paying for the snake. Did you do any research on the snake at all before you decided to buy it ? Information on the wobble in spider's and spider morphs is very well known and easily accessible.
While it can be extremely unsettling to watch, most of the time it is nothing to be worried about. These animals can still eat, shed and grow normally. Over time the wobble may get less severe, or it may not.
If you are that bothered by it, I would return the snake and look into getting a different morph.
Gale