These are actually things you need not bring in great quantity (and things that I stock up on in Seoul). Socks you can get on the street or in the Namdaemoon and Dongdaemoon for about 500won/0.45USD to 1,000 won a pair. Underwear you can get 4 for 10,000 won/9USD or lless (for boxers, at least). Belts I get from a travelling belt guy but there are many sellers in the markets too.
Toiletries and Medical Supplies: Near exits #13 and #14 of Dongdaemoon Stadium Station (lines 2, 4 and 5 intersect here) there is an old lady that sells disposible razors 10 for 1,000 won/0.90USD. Yes, 9 cents apiece (you can use them 2-2 times on an average beard, too)!
Miscellaneous: More shopping stories here and here,
Luggage and bags:
Bring some anti-bacterial toilettes and a little pack of toilet paper with you when you are touring around town. They come in very handy. I recommend this for anywhere in S.E. Asia.
Toiletries and Medical Supplies: Bring some anti-bacterial toilettes and a little pack of toilet paper with you when you are touring around town. They come in very handy. I recommend this for anywhere in S.E. Asia.
Miscellaneous: - Get a Korea passcard. They are like pre-paid credit cards, they come in four denominations from 50,000 won (about $40) to 500,000 won. They can also be used on the subway, Airport Limousine bus, and as calling cards. The best part is not having to carry cash (except for market visits), and if the card is lost or stolen your funds are protected. As soon as you exit the immigration doors, look to the left for the passcard desk next to the information desk.