The Mishnah in Sheviis 7:4 seems to imply that it was fairly standard for people to buy a bechor (baal mum, obviously) from a Kohen to serve at special occasions, like Yom Tov or a wedding.
Is that practice mentioned more explicitly anywhere? I'm interested both in the times of Chazal and more recently. Does anyone give a reason (like if you can't give it as a korban, at least use it for a mitzvah)?