Liver is so healthy if you get it from a good source. I'm not sure I'd want to buy liver from any old supermarket. I went looking for it at Whole Foods once and they didn't have it. :/
I googled a little about liver concerns. You don't really have to worry about antibiotics, because they end up in the fat stores of animals. A possible issue would be metals, since the liver filters those out of the blood and there might be a buildup, but is there a difference in the metal consumption of conventionally farmed cattle and ethically farmed cattle? I don't know.