I mentioned earlier that Cain was a crop farmer, and Abel was a shepherd. Now, at the time of the murder, they offer a sacrifice to God. But, was this the first sacrifice? It seems unlikely. They’ve been around long enough for Cain to get married, so it’s very likely that they knew about sacrifices. In fact, it seems likely that there had been some bartering in the past, with Abel selling sheep for some of Cain’s produce.
But, whether this is a new thing or old news, it’s clear that God wanted animal sacrifices. Only blood can cover for sins, not vegetation. So, Cain’s offering doesn’t measure up to God’s standards.
What he SHOULD have done is trade Abel some produce from his farm for a sheep or other suitable animal. For some reason, he didn’t. And, far more importantly, God didn’t view it as acceptable.