I think that automotive-wise, it is a buyer's market. You can get great deal on vehicles 3/4 of the time. Of course you have to bid within your geographical region so you can go and look at the car in person, unless it's really really new and they have really really high feedback, and even then, I would look at it.
As for little things, like LEDs and cells and books and such, it's also a great place. You have to make sure you getting something from the seller with a high enough rating.
A lot of times though, I couldn't find some specialized items I was looking for, for example some 12,000 MaH D cells, all they have are some 10,000 MaH ones. Then I go to an online place. But I did get a bit lot of C cells for a huge discount.
One huge advantage is that you don't have to stand in line in some noisy Walmart-like place with questionable people around you (sometimes) and drive there and whatnot.. (Not to mention exposing yourself to accidents and crime in the parking lot, crime, theft of your vehicle, etc.) I've purchased things on there even when they came out to about the same as in Home Depot/Chimart - it's nice and raises the feedback. Ebay generally has much greater selection than anywhere else. I even stopped going to yard sales which I previously liked.
Some problems I've had:
With clothing, you have to know your size. Even then, sometimes an item will not fit perfectly.
Some things are just hard to buy, like eyeglasses frames. You have to know your size, but it's inherently difficult. But if you do get it right, you don't have to pay crazy prices like $200 for a Chinese-made Titanium frames... Get it for 1/4 of the cost if that.
Shipping can be too much with big items like computer monitors. I got a nice monitor for $25 (which cost $500 new, but had a few scratches) and shipping came out to $60. So big and heavy items like TVs and fridges are not very practical -- but even with shipping, may turn out to be a better deal than the local hardware place.