Depends on the perspective you take. If I have to free up funds urgently for a bargain purchase, I don't mind selling off names for a small loss or what I paid for them etc, as I know it is countered against the profit I will make on the name I am buying.
Also, many times I have sold early, or flipped a domain, whcih I could have got much more for if I held out, but often this is the way to keep moving up in this industry. I've also dropped a fair few, but I wouldn't count that as a loss, more just part of the way it works.
Like Sharon as well, I have bought many traffic names which I have sold for the same or less than I paid when the traffic has died, but they've always made me a good profit when factoring in the ppc revenue I received.
In summary, I think swallowing small losses is often a necessity and you need to put it in the context of your overall business plan.