Is the witnessing of a sale required in the verse (And have witnesses when you trade with each other) specific to certain types of sales, or is it abrogated, given that people do not witness sales in marketplaces?

The witnessing of a sale mentioned in the noble verse is understood to be for recommendation and preferability, not for obligation. It is recommended in all contracts of sale, except for the negligible few for which it is not customary to document with witnessing, such as buying bread, vegetables, water, and similar items.