Originally Posted by TrainsRMe:
Singles, definitely. When I was avidly collecting, I often wanted one piece of a lot, but didn't have the $. Also, if one does have the means to get the whole thing, they're then stuck with having to sell any unwanted pieces themselves. You'll realize more money from individual sales, as well.
In the case of 12 cars where the only difference is the number, it does make a lot of sense to be willing to sell any or all of them. In the case of something that really is supposed to stay a "set", like the Lionel "Northern Pacific Freight Set" or one of the 6464 box car sets it can make it significantly harder for the seller to sell the rest of the cars.