I've found that price differential quite a bit on different car parts.
Rock Auto is another good source.
That said, the brick and mortar stores do have overhead costs they have to pay (salary, rent, electric etc..). Amazon does too of course, but the scaling is different.
Was that $120 after the 20% off coupon you can routinely find online?
As far as Amazon switching to inferior/knock off parts, I've never seen that happen unless they paint them up to look like the real deal. Not saying it doesn't happen, I just haven't seen it.
I try to support my local AA store, especially if I need/want it today and they have it. After all, if online takes over the world, the local stores will be gone.
Mike