Hilchos Shemittah v’Yoveil 6:2

One may not harvest vegetables from a field and sell them during shemittah, nor may one charge for dyeing using the shells of shemittah produce. This is considered doing business with shemittah produce. If someone harvested vegetables to eat and he had some left over, he may sell what’s left and the money received is treated like shemittah produce. Similarly, if someone harvested vegetables for himself and his son or grandson sold some of them, the sale is permitted and the money received is treated like shemittah produce.

Hilchos Shemittah v’Yoveil 6:3

When shemittah produce is sold, it may not be sold by volume, weight or count; this is so that it won’t look like one is selling produce during shemittah. Rather, one may sell a small amount by estimation, making it clear that the produce is ownerless. The money received must be used to buy other food.