How can I receive a large parcel in Korea when my building has no reception desk?

The first thing worth checking with the seller is whether the item ships as an ordinary courier parcel or as freight, because the two behave very differently. Parcels from CJ Logistics, Hanjin, Lotte or Korea Post are usually just left at the door when nobody answers, which in a building without a reception desk is exactly what you want to avoid. Bulky items like furniture or appliances often go by freight instead, and in that case the driver calls ahead and needs someone to receive it, so you get to pick the date.

For normal parcels the simplest fix is the delivery request box at checkout. On Coupang, Naver or Gmarket there is a field called 배송 요청사항 where you can type something like 부재 시 무인택배함에 넣어주세요 if your building has a parcel locker. Quite a lot of one-room buildings in Daejeon do have a small locker bank near the entrance even without any staff, so it is worth walking the lobby once to look before you order.

If there is no locker, convenience store pickup is what most students end up using. GS25 and CU both hold parcels for a few days and you collect with your phone number, and you select the store as the delivery address when ordering. The size limit is modest though, roughly 60cm and 20kg, so it works for clothes or electronics but not for a desk or a mattress.

For anything genuinely large, tell the seller before you pay that you are in class on weekdays and ask whether the carrier can call you the day before to agree a time. Korean sellers get this request constantly and it is rarely a problem. If the driver calls in Korean and you get stuck, the free BBB Korea interpretation line at 1588-5644 works well on speakerphone, and the 1345 immigration contact centre can do a three-way call too. A two minute delivery conversation is exactly what those services are for.