I will chime in on the side of "fixed price". It is purely anecdotal, but I have had much better success with fixed pricing, not only increases in total sales, but higher average per sale as well (almost double - maybe I'm just a terrible negotiator). A lot does depend on the types of names you are dealing in too. I buy mostly hand reg, dropped, or inexpensive auction names that are geared toward individual or small/home business and fall in the mid to high XXX price range retail. I feel I have a good idea on price point and what I am comfortable selling at to avoid seller's remorse.
I have some names where I feel the end users would likely be large companies - those I leave as negotiable.