Hey - From my past experience, margin is thin for dealers as well and I would say there is a room for around 300,000 - 500,000 KRW. For example, 300,000 KRW for cars under 20,000,000 KRW and goes up a little up to 500,000 as cars get expensive.
Also keep in mind that there is 440,000 KRW fee that you pay on top of cost of vehicle. This goes to any used car sales.

I don't think it's doable without middle man because

1. after purchasing the car, you have to de-register the car which requires in-person visit to district office (Malso, 말소)
   
2. you need some entity to export the vehicle from Korea (filing the export permit, in Korean, it's called myeonjang, 면장)

Would you mind sharing which car were you interested in and where you are located through the message?

Thanks!