A liquidator is a business that sells merchandise that is somehow not suitable for sell at a normal retail outlet. There are various reasons that it is not sold normally: it is sometimes overstock or a discontinued item, or it is factory second material, or it comes from a failed business or closed down branch. Some liquidators also sell the equipment that offices and other businesses used for themselves. Sometimes it is perhaps not too good of an idea to ask too much where a liquidator's merchandise came from. Just as there is different sources for the material, the material itself varies widely. Some liquidators carry mostly small consumer goods, hardware and office supplies, while others specialize in larger, more expensive items, like home or office furniture. Of course, many have both; the word "specialize" and "liquidator" don't really go together, as such. The one class of items that don't show up quite the same way at liquidators is food, because food doesn't quite survive being found damaged five years late in a warehouse the way a pallet of Barbie trapper keepers do. Most liquidators will have a small assortment of smushed cookies and candy, but liquidators that specialize in food prefer to call themselves outlets. What I say about the business practices of liquidators may be colored to some extent by my geographical location: it is possible that liquidators in other parts of the country may have slightly different business models, but probably not by much.
The basic economic picture being painted (and it is pretty basic), I should add that liquidators, if done properly, have a qualia all of their own. They are also one of the coolest places to shop: and I am saying this from the viewpoint of 2008, when buying money on junk to be ironic has already expired as an interesting past time. If shopping is an interesting experience however, shopping at a liquidators is much more interesting than shopping at a Dollar Store, which mostly have tacky plastic things, and may or may not be cooler than shopping at a thrift store. The reason for this, as I said, perhaps has something more to do with the qualia of a good liquidator, rather than any specific differences I could name. Some of it is explainable: the clutter, the density, the mixture of products old and new, useful and stupid. But above it all, liquidators just feel like a Sunday afternoon in a garret. And if you don't believe all of that poetical nonsense, you can at least trust me that a liquidator is a good place to find a 36-piece screwdriver set and a package of two dozen misprinted pens for under 10 dollars all together.