Two unrelated things sound alike. 사고팔다 is the compound 'buy and sell', used for property and secondhand goods (집을 사고 파는 부동산, 개인 대 개인이 사고 파는 거). But -고파 is a contracted, soft form of -고 싶다 'want to': 보고파 is 'I miss you', 산에 가고파 is 'I want to go to the mountains', 이어가고팠던 is 'that he had wanted to keep going'. If a noun object precedes it, expect the buy-and-sell reading.
Register: -고파 is lyrical; common in songs and captions