“Jews don’t buy retail”?
we have a 1/6 measure for overcharging. Both sides of the deal are in symmetry here. That is, a buyer also can not press or use ignorance of the seller to give him too low a price. It does not matter whether you are dealing with a billionaire or a corporation. Of course, when dealing with a professional or corporation, they are usually informed and not desperate for a deal, they most likely have a lowest number that they can agree to. They may also be paid on commission. Maybe a criterion similar to returning lost objects might help – if a sum is small enough not to matter to you, you should let it go. Longer term – it is better to establish a “fair price” that works for both sides than press your advantage. I found this works well with some school financiers.
Not sure with non-Jews: you don’t want to give them extra funds for trick-and-treating, but it does not mean you should not let him earn a living also. Again, a fair price for both sides may make him into an ohev Isroel.