First, check the seller's track record. eBay makes that easy to do.
Second, if it sounds too good to be true it probably is.
Third, many eBayers sell what is referred to as "grey market" cameras and lenses. They may or may not necessarily point that out in their ads. Grey market are usually imports and come with NO manufacturer's warranty whatsoever. Some folks think they are worth the risk. I'm not in that camp unless it's a fairly low-dollar item.
Fourth, stop listening to Ken Rockwell (Although on this one he is correct. Every clock is right twice a day).