There are numerous stores scattered around Korea; obviously a few in Seoul, but also in other locations. There are several chain stores that sell their own branded versions of ego style batteries, but they're usually very expensive. Online stores offer better prices on similar equipment. Google ecig korean forum, head to the .co.kr site and click on the banners for a few online stores. They don't usually stock high end mods, though you'll find a couple of clones. RBA supplies can be found here, as can VG, PG, and flavors without nicotine are all online. Nicotine can be purchased from B&Ms, but can't be sold online.
The difficulty will be your level of Korean, and how well setup you are here. Despite having some of the fastest internet in the world, shopping from Korean online stores can be a pain. You'll need your Korean social security card (foreign equivalent), mobile phone, etc. Even then, some stores have other methods of verifying ID that involves an online PIN system linked to your social security number. Forget about online payments, which is easy enough to do because most websites offer bank transfer, which can be done at an ATM. Gmarket can sometimes be a good source of supplies.
Having said all that, in terms of DIY liquid etc., it will also depend on how comfortable you are using ingredients from an unknown source. Researching the type of VG, PG, nic, and even flavoring being sold is difficult even with a decent level of Korean. Distilled water and alcohol can be purchased at most pharmacists. The DIY flavors are okay, but simple fare. You won't find many complex blends, if any.
Any import (of any product) will be taxed if it has a value of greater 150,000 won, at around 15-20% depending on what the item is. You are right re. nicotine. Nicotine liquids are taxed at a flat rate of, if I remember correctly, 400 won / ml. You also have to pay a 'local education' tax. This is regardless of total cost or amount of liquid. I got caught on a Halo sampler (35ml total) and was charged a total of 21,000 tax. Which, despite bringing the total cost to about $40, would still be cheaper than buying a Halo liquid at a store here (60,000 won for 30 ml). Having said that, I wasn't picked up on a recent order of about 150ml from Giant Vapes. The parcel was labelled as 'flavor concentrates', which may have helped, or I may have just been lucky. Hardware does not get taxed unless its value crosses the 150,000 won import tax threshold.
The best way to have a friend send you liquids, as far as I can tell, is to have the value including shipping listed as less than 150,000 won, and label it as something appropriate (flavor concentrates, essential oils, etc) that does not involve language relating to nicotine or vaping. Your friend may be able to re-bottle the juice in unmarked bottles, as it's not at all uncommon for customs to open imports.
So, after all that, yes; it's entirely possible to equip yourself with everything you need to vape, all sourced locally. However, if you're accustomed to a certain standard of vape, prepare to do some work, or simply import.
