Buying Used Hassy
I'd like to cast my positive vote for KEH. They rate very conservatively, and they have a return policy, which I have never had to use BTW.
I would like to add to Victor's comment about most Hassy gear being used by professionals. I would agree, but there are also items that have been purchased by affluent amateurs. Amateurs will coddle their gear, meaning it will look good, but are much less likely to have it serviced regularly, meaning it may need an urgent CLA.
A CLA for a lens in the US is probably around $150, assuming nothing is broken, so I would look for perfect glass, and mentally add $150, unless the seller has a documented recent CLA. Marks on the barrel seem to be the norm for older chrome lenses, and do no harm, but any dents should be an instant red flag. The only way I know to dent a Zeiss lens is to drop it from a fair height onto a hard surface...
That Sonnar of Victor's is gorgeous. Sure, you might find the same lens for a couple of hundred less that what he was asking, but when you factor in a CLA you may be saving a just small amount of cash and buying into a heap of uncertainty.
So I'd say buy from a trusted source and expect to maintain your gear, just like you would your car. Unlike your car, though, your Hassy gear will never depreciate to zero!