You can place nectar racks on community lots, and set them to allow sims to taste (and buy) nectar: In Build mode, SHIFT+CTRL click the rack. You will have several options, two which are "Make Tasteable" and "Make Buyable". You can then use "Treasure Component..." to choose which type of nectar you want in the rack, either one-time or regular (which is best for a bar). About 3/4 of the way down the list that pops up are the nectar choices: you'll probably want to use either "Nectar Bottle - Cheap", "Nectar Bottle - Medium", or "Nectar Bottle - Expensive".
As for selling sim-made nectar in a bar, there is no way that I know of doing this.