Restaurants sometimes will buy the cellars of collectors. The advantage is that they will often take the whole cellar, so that the seller is not left with some more undesirable wines. The disadvantage is that the total sale will be less than if the seller had gone to auction: convenience versus profit.
Some full service wine brokers will also buy your wine in small or large parcels. Vinfolio (www.vinfolio.com) is a good example of this. They have an inventory of your wine, stored in their facility or elsewhere. They will sell the wine for you as one of their services. They will either wait for a buyer for the specific wine or they will buy it from you and then either sell it to other Vinfolio customers or at auction. The latter option will give less return to the seller as again it is more convenient.