If you've lived in Korea for any amount of time, you'll notice most things are overpriced, especially if you hit up a Lotte department store. You've got to check the internet for normal prices.
I've been using GMarket with no problems. They have an English site that you can register at. You'll need to search on the main Korean site for what you want. Searching through the English section gives strangely limited results.
If you can read a bit of Korean, try Danawa.com for your electronic needs. I like wandering into the Yongsan electronics market so I just check prices there before I go. But you can buy online too.