OK, if you must have a specific item or service today, then your bargaining power is significantly reduced. Your BATNA (best alternative to a negotiated agreement) is to pay full price from the specific person you're talking to. If your seller detects that through your words or actions, you will always pay full price.

I'm saying that being able to walk away (even if you don't) is your most powerful piece of negotiating leverage. If you go into a negotiation without the ability to walk away, you're in a very weak position, indeed.