Is describing a vendor as a "thief" in front of people, with the intention of referring to the shop and not the person, considered an injustice that requires seeking his forgiveness, especially given that he is not of Arab nationality and it is unknown whether he is Muslim?

Describing the seller as a "thief" is an injustice. It is either backbiting (ghībah) if what you said about him is true, or slander (buhtān) if it is not true, and both are forbidden. You must repent by feeling remorse and resolving not to return to it. Seeking forgiveness for him (istighfār) is sufficient, and you are not required to seek his direct absolution (istihlāl) to prevent greater harm.