Tipping is not required nor expected in Korea. But most major hotels add a compulsory 10% service charge to bills. This is on top of the 10% VAT (which is usually included in prices at most stores in Korea, but not in some high-end restaurants). Taxi drivers will appreciate it if you tell them to "keep the change" (or jandon gajiseyo in Korean), but this is not expected and they will have trouble understanding if you want to give them anything more than change (like "keep 1,500 won and return only 2,000 won to me".)

You can barter in the open markets for lower prices, but make sure you do so politely. Of course, bargaining is becoming a rare sport as most stores now have fixed prices.