I think the most commonly used online shop, especially among expats here, is gmarket. Virtually anything you need to buy, you can purchase through gmarket. Gmarket is especially convenient because it is one Korean online site that offers English navigation.
However, gmarket doesn't have everything and they may also not offer the best prices. When shopping online, I use gmarket for most general things I need to buy (trash can, towels etc.), but when I'm looking for something more particular, I like to search other specialty online shops that have a smaller, more curated collection of things. For example: 10x10 sells modern, unique and well designed products.
If you shop around via 10x10 first, you're likely to find what you're looking for a lot quicker than if you were to do a broad search on gmarket. However, did you know that most products you find online are sold through a whole host of other sites? (I don't know why it took me so long to figure this out)
For example, while shopping for a lamp, I checked out 10x10 first because I knew I wanted a certain aesthetic. Shopping through 10x10's smaller collection, I found a lamp I liked. Originally I thought that this lamp was solely sold via 10x10, but this isn't the case 9 times out of 10.
So, in order to get the best price on any item, I recommend doing a quick naver search.
1. Find the product you want.
2. Go to Naver.com, and search the product name. The product name will most likely be written in Korean, so you need to also search the product name in Korean. If you can't copy and paste from teh original site or you're computer can't type hangeul, use this website to string along the Korean letters, then just copy and paste into Naver's search bar.
3. Look under the shopping tab function and click 지식쇼핑 더보기
4. This will bring up pages of online sites that sell the same product. The links and prices are clearly shown.
This process is also convenient if you are a sole gmarket shopper. If you can only navigate through gmarket's english site, or if you don't want to go through the hassle of creating an account for another online retailer, this is a good way to try finding your item via gmarket.
Online shopping is definitely the way to go in Korea, especially with their speed of delivery!