If it is a bubble, that's not a defect. Bubbles used to be very common in lenses; some manufacturers stated in their materials included with lenses that bubbles were normal and would not affect picture quality. Later production methods reduced the incidence of bubbles.

Same with dust. I have seen a statement by Pentax that some dust might appear in their lenses over time, and it will do no harm.

If you're unhappy, return it. That's KEH's policy and their attitude. I once showed to a KEH rep. at a camera show some fine scratches on a front element which I did not see initially (and which they had missed). Even though it was out of their warranty period, he told me to go ahead and return it, and tell them he said so. They deal with so much stuff I think they'd rather have a happy customer than quibble.