Most recently, my husband purchased me a fairly new treadmill that looked like it had hardly been used. Of course we don't have any type of warranty on the machine, but even if I get a year out of it, I didn't have to sink close to a thousand bucks right away on one. There is also this rumor that treadmills are the most desired but least used thing people claim they want!
There are items that I would never consider purchasing used or from a person rather than a store. But, if you are willing to buy it at a garage sale there is no reason you can't turn to Craigslist to find it. This is more convenient to all parties as well because garage sales are in a specific location at a specific time on a specific day. On Craigslist you can arrange a place to meet (depending on the size of the item being sold) and the day and time can be flexible based on every one's availability.
Another great Craigslist find is furniture and musical instruments. There are people all the time down sizing or changing the style of their homes, so people are willing to part with nice, quality furniture (sometimes antique) at a very low cost.
Musical instruments are much like a treadmill, people think they have an interest then they realize they don't have time or don't like it and want to get some money out of an instrument they bought new. Sometimes, you can get a good deal from a musician looking to upgrade his own instrument and needing a little extra cash to be able to purchase it.
Craigslist also offers the possibility of trade, or bartering. You can list an item in that section and specifically request what you want to trade it for, or leave it open and be surprised with the offers you are made.
There is a show that is fairly new on t.v. called Barter Kings, they start with one item and "trade up" until they reach a specific monetary goal or have the item someone else wants in a trade for something they have been eying.