As the markets continue to dive, it's getting more and more important to make a decision on how to handle selloff/sell-off/sell off.
It looks like The Wall Street Journal (see the headline above) and Bloomberg prefer selloff, though neither is consistent.
The Associated Press seems to to prefer sell-off. That makes sense, as there's no stylebook entry and AP refers to in such cases to the Webster's New World College Dictionary, which lists sell-off. But AP has some stray uses of selloff.
The incredibly well-edited New York Times remains unfailingly consistent in its use of sell-off.
Mr. Rewrite nudged the AP Stylebook's Twitter account for direction, but no word yet. Until then, he's sticking with sell-off.