Selling an animal to butcher will break first precept? Farmers who breed animals and sale to butcher or for food concern is considered as intentional killing and break first precept?
With regard to precepts, one should not kill or encourage others to kill. If one is supplying animals to a slaughterhouse one is encouraging others to kill the animal though one is not participating in the act. So this breaks the present.
The conditions for killing are:

i) The being must be alive.
ii) There must be knowledge that it is a living being.
iii) There must be intention to cause its death.
iv) Action must be taken to cause its death
v) Death must result from such action.
If all these conditions are fulfilled, then the precept has been
broken.

FIVE PRECEPTS (PACASILA)
When one is trading in livestock, the animals are alive and one knows the animals are indeed alive. One knows the fate of animals sold to the saluted house. Knowing that if one giving an animal for slaughter, by the act of giving one is condemning the animal to its ultimate fate. By doing so one seals the fate of the animal. In the slaughterhouse, the animals are ultimately killed.
Also, Right Livelihood component of the Noble 8 Fold Path says one should avoid 5 trades:

A lay follower should not engage in 5 types of trades. Which five?
(1) Trading in weapons sattha vaṇijjā,
(2) Trading in humans satta vaṇijjā,
(3) Trading in meat maṁsa vaṇijjā,
(4) Trading in intoxicants majja vaṇijjā, and
(5) Trading in poisons visa vaṇijjā.

I guess trading in livestock may be classified under trading in meat as ultimately this is what happens when you sell animals to a slaughterhouse.
One can accept “available meat” (pavatta,maṁsa) which is per killed not specifically with oneself in mind