At the end of the day, before starting the negotiation you need to know what your best alternative is. Then it's just a question of entering a discussion to do better or equal that. If the shop won't match your best alternative by exercising their right not to sell, you exercise your right to buy the best alternative. Very simple concept.

Question is, what is your best alternative and how much does it cost? If comparison is difficult, e.g. you expect to receive future better service from your local shop, you will need to place a $ amount on that service. Is it worth $0, $10 or $100? There is no right answer, but if you factor that in you will at least have a firm basis for negotiation.

Next question is how to go through the discussion. Again, no right answer, but it helps if you speak to the a salesperson who is in a position to do a deal not the saturday workshop help. Next convince the salesperson that you are a serious buyer not a timewaster. Normally this means having a clear idea of what you want and sticking to it. Once you've communicated this, you can ask whether the salesperson is flexible on price. Normally they will ask you to say what you're willing to pay, at which point you have your best alternative ready to go. Normally I would start with a price a little below the best alternative and go from there.

I find that if I've done my research, it pays to reveal a number close to your best available alternative early in the negotiation, since the salesperson then knows you want to buy, and doesn't need to spend 1/2h horse trading with someone who has an unrealistic idea of what an item costs.