What is the ruling on repeatedly looking at a veiled woman who has uncovered her face for the purpose of buying and selling?

If a man needs to repeatedly look at a woman's face and hands without lust, due to the necessity of dealing with her in buying, selling, and similar interactions, there is no harm in that. The gaze should be limited to the extent of the need. Jurists have permitted repeated looking for necessity, as in cases of bearing witness or proposing marriage, and the governing principle here is necessity.