Itaewon is the area near the military base in Seoul.  South Korea has benefited greatly from increased tourism, and prices are not what they were 20 years ago, so don't expect to pick up quality leather jackets for $20. If you're not willing to pay, the next tourist will be, so they won't sell to you. You can pick up a very good quality, custom-made leather jackets for $100 with little or no bargaining though.

Furniture, if you're in a position to buy  and transport it back, can also be an incredible buy, and is negotiable, up to 30% off. Be careful on quality though. If you live in a dry climate, your Korean furniture will very likely crack when you get it home.

Jewelry is one area where where it's easy to get ripped off as prices are inflated and standards may not be up to par. While gold and silver are okay to purchase, white gold is tricky.

Custom made clothing can be wonderful for men, but beware of the guys touting their wares on the streets. The good ones don't have to try to pull in customers from outside. Just explore some of the alleys off of Itaewon and find someone you like.  For female clothing, if you're not concerned about getting clothes tapered around your bust and hips, buy away.  A good idea is to have them make something cheap first, like a tailored shirt ($20), and see if they can.

DVDs, especially those that have already been released, are between $5-$8 and generally of good quality. Make sure they show you the movie playing before you buy.

Korea's native craft is celadon pottery. Beautiful pieces can be bought anywhere. Just shop around and pick what you like.

Purses are an awesome buy and of extremely good quality. Leather purses are anywhere from $30-$100, and imitation cloth bags from various designers are $15-$25.

Haggling is not impossible. However, you ought to know that if you're the first person in the shop in the morning, your performance may well be regarded as setting the tone for the day. If you haggle too hard, your shopkeeper may get upset, and if you walk out without buying, that may be taken as meaning the day will be without profit.  Answer: don't be first in. Or buy something small and cheap if you are first in.