My own dealings with yahoo auctions have mostly been excellent.
There are hundreds of categories with thousands of items for sale. The site is laid out very well and is very easy to use.
They have a rating system where you can rate someone you have dealt with, so if you got a good deal you can give that person a good rating. In the same way if you had a bad experience you can give the person a negative rating. This way you can tell what a person is like before you buy from them!
The only problems i have noticed are that alot of companies advertise through auctions like these which can be very misleading - imagine when the bargain you were looking for turns out to be a shop and there is no real item up for auction. This is the only problem i have seen, but you can hardly blame yahoo for that, although i think they could monitor repeat auctions.
On the whole though - worth a good look!