The low retail minus a few thousand...or better search the web for similar units and see what the asking prices are running (then subtract a bit off that). Certainly start with low offers. If you go high, you can't cut it later.
If you are worried about offending, do an oblique offer: "Gosh I really like her but I only have $X available right now."
If you are way below, he isn't going to get in your price range anyway, so who cares if he is offended but now, you have started the price discussion. If you are close, he likely will counter offer either right then or in a day or two....maybe a couple weeks later when he gets no bites (make sure he has your number before going to look at it).