(Exodus 21:37) "If a man steal an ox or a lamb": Both the slaughterer and the seller were in the (general) class (of thieves), viz. (Ibid. 22:6) "if the thief is found, he shall pay double," and Scripture removed them from that class to make them liable for the more severe "four and five" payment, wherefore this section was stated.
The Torah Deliberately Removed These Two Cases from the General
Mekhilta Tractate Nezikin 12:17