Simlim shops are generally quite rude (some are nice only to chio girls - I observe but that is only done by youngster sales guys).
Only info they give you is price, if you ask too much then they expect you to buy. If you dont buy they are not happy. Stuck up.
So I did all my research on the internet and narrow down my choices (and rank them). Then I visit shops to ask if they have stock of items I want and their best price. Sometime there is no stock but they display their price (I believe this is can be their tactics discredit competitor who has stock but list higher price), and they offer another saying this is similar. Don't fall for that tactics. You can wait for the price to go down, but do note price fluctuates because of exchange rate also.
You must know what you want and stick with that.
Generally shops at higher level are selling cheaper, shops behind (the alleys) are also cheaper.
Make sure you check for all accessories. Accept only sealed boxes, no damage to sticker. Tell them not to open for you, and if they do you will not buy. Some play play pretend to open, but it it actually already opened (I can see the seal got dust - if just opened it cannot be the case).
Make sure you have the receipt, it say the correct thing you buy (model number matching etc), and check the warranty card (has to be chopped).
If you have problem with short term memory or easily overwhelmed by brigth light and info overload, bring a friend who does not and tell him/her to remind you about these things.