준비하다 means "prepare, make preparations (for)" 마련하다 means "make an effort and make preparations (for)" e.g. 나는 선물을 준비했다 I prepared a present 나는 선물을 마련했다 I made an effort finally prepared a present 외출을 준비했다.(O) I'm ready to leave 외출을 마련했다.(X)