Surplus is a loaded word, especially when shopping for electronics. To some, it means junk that just fills up the garage, like musty canvas tents or funky fold-up shovels that the US Army used back in the 1950s. To the true electronics buff, surplus has a different meaning: affordable components that help stretch the electronics-building dollar.
Surplus just means that the original maker or buyer of the goods doesn't need it any more. It's simply excess stock for resale. In the case of electronics, surplus seldom means used, as it might for other surplus components, such as motors or mechanical devices that have been reconditioned. Except for hard-to-find components — such as older amateur radio gear — surplus electronics are typically brand-new, and someone still actively manufactures much of this equipment. In this case, surplus simply means extra.
The main benefit of shopping at the surplus electronics retailer is cost: Even new components are generally lower priced than at the general electronics retailers. On the downside, you may have limited selection — whatever components the store was able to purchase. Don't expect to find every value and size of resistor or capacitor, for example.
Remember that when you buy surplus, you don't get a manufacturer's warranty. Sometimes that's because the manufacturer is no longer in business. Although most surplus sellers accept returns if an item is defective (unless it says something different in their catalogs), you should always consider surplus stuff as-is, with no warranty implied or intended (and all that other lawyer talk).