in 应该是it才通顺
因为有even if 所以后句为虚拟语气 had they spent the moeny themselves.是 if they had spent the moeny themselves的变体 if省略后 had提前
句子为Even if a gift is enjoyed, it may not be what the recipient would have bought if they had spent the moeny themselves.
所以句子可以翻译为
即便礼物很令人满意,如果是收受礼物的人自己掏腰包,礼物也不是接受礼物的人想买的.