It is permissible, if both the customer and the seller are pleased with this. Allah The Almighty commands:
“Oh you who believe, do not consume one another’s wealth unjustly but only in trading by mutual consent.” (An-Nisa/ The Women 4:29)
Buying sacrificial by kilos is not similar to buying meat from butcher. In this case, an agreement is made to have a sacrificial slaughtered. The essential point of sacrifice is “shedding blood (of cattle) by the intention of worship”. This condition is satisfied by the mentioned trading.