Bekhorot 32: three things to remember

Young orphans may sell blemished animal-tithe meat normally, whereas adults may only bundle its value into hide, sinew, or bones.
The Torah's phrase 'it shall not be redeemed' forbids selling live animal tithes and entirely prohibits redemption.
Beit Shammai restrict a blemished firstborn to priests, but Beit Hillel allow non-priests and even gentiles to eat it.