Is there a prohibition against selling items that are usually weighed by count (such as grapes by the piece) or items that are usually counted by weight (such as watermelon by the kilo)?

It is permissible to sell grapes by count, measure, or weight if the price is in cash. It is also permissible to sell them for another usurious commodity, such as dates, provided there is immediate mutual possession. Or for a non-usurious commodity, such as a garment, without requiring mutual possession. However, selling grapes for grapes is absolutely impermissible, because it is a usurious commodity that requires complete equivalence, which cannot be achieved by count or weight due to the variation in grape sizes or the change in their weight when dried. Similarly, it is not permissible to sell fresh dates for fresh dates unless they are dried and sold by measure or weight, based on the hadith (definition): "Gold for gold... like for like," and the hadith in which the Prophet, peace and blessings be upon him, forbade selling dates for fresh dates.

As for fruits that are eaten fresh, it is absolutely permissible to sell them for cash. However, the more correct view is to prohibit selling some of them for others (such as one watermelon for another) due to the difference in moisture and weight when dried, and the variation within them that prevents the achievement of equivalence.