Define "user condition?" Well, let's see. . .
The Leica collector's definition: The lens was removed from its plastic wrap and handled with cotton gloves. However, there was a hole in one of the fingertips, and a small amount of skin oil touched the surface of the barrel, thereby despoiling its chromium virginity. The lens is now worth only half the annual gross national product of San Marino. A pity, since, if mint, it could have been traded for the entire Vatican Treasure, plus the hidden Swiss assets of a number of defrocked Fortune 500 executives.
The eBay definition: The lens was used everyday by a reporter who covered Bosnia, Afghanistan, Iraq, the Paris riots and Boston's Big Dig. It has been dragged several kilometers by a tank in an unpadded canvas army bag, taken several shrapnel hits, and dropped over a cliff at Tora Bora. The focusing helix is well-damped with sand and fine silt. There are several pits in the front element, which do not affect picture taking quality. Only 10% of the original paint is left on the barrel, lending it an attractive patina. The diaphragm is smooth and snappy, with a fine sheen of oil and a trace of blood on the blades.
But seriously. . . if the glass is clean, and the focusing is smooth, a little dust won't hurt anything. A lot of dust means you'll probably need a CLA, and you should bear that in mind when you set a price.
The other responders have already given you some good quick tests.
--Peter