If you know what you are looking for, and know how to ask the right question. Any misunderstanding can be avoided.
Many a time, the customer have unrealistic expectation on what they are buying, and to make matter worst, they bargain like tommorow do not exist. When all is settled, the customer regrets his decision a while later. So he try to salvage his own situation, by "creating" some senario to back his plan of recourse. I have seen this happen too often, end up un-nice things being done, and everyone not happy.
As a customer, you have to depend on your street-wiseness and common sense. Would you continue to pursue if you feel a bit amiss during the sale or do you back out??
Plan your purchase in advance, how much effort are you yourself willing to put in. Make it a point, that you know what you are getting into, before plionking the money on the counter. Like we say "Prevention is Better than Cure".
My own method is to find out about the product as much as I can, so I don't need to rely on what the salesman tell me. I find my ideal price, and walking into the shop, no time wasting, just say "I want THAT, what price??". If price is ok, I just say "OK" and inspect my buy before plonking down the money on the counter. Afterwhich just say "Thanks" anyway to develop soem goodwill. And leave, and DO NOT PURSUE further checking other shops on the same item anymore. Just go back, open the bag and be glad with your purchase. Most of the time, I spend 5 minutes or less in any shop for the transaction. My method is called "FIRE & FORGET!!"