Just say the prices are set, no discounts. If you want the plant/s, then they cost that much. They won't find it cheaper at a pet shop. So they either pay your price or go somewhere else.Anyway, I just successfully sold an Echinodorus and a bunch of used aquarium equipment on facebook. For the questions, when it comes to people haggle over the impossible cheap price, how should I deal with them? Espescially in real life situation?
You can also say you are selling them for someone else and the price is set by them. You have no say over the price because they are being sold for another person, and that person wants x amount of dollars for that item.
Don't invite people around to your home (or give them your address) unless a price has been set and accepted beforehand.
Don't show people around the property and don't show them plants or other things you aren't selling. Have a container of plants out front or in a bag by the front door. When they arrive, get the money, hand them the item and thank them, then close the door.
If people spend a lot of time around your place looking at unusual plants, they might come back later and steal things.