I've had a bad experience in this case, that is why i now rather spend a few extra bucks than waste my money.
Back in 2004-5 i fell in love with 212 Men, i was in Karachi on holidays then, went to a very famous shop by the name of Naheed, big bottle was costing 5 or 6k if im not mistaken, didn't buy it right away because i wasn't an impulsive buyer back then, didn't have a job was just striving on pocket money perhaps that is why lol.
Anyways the same night i went out to buy some clothes in another local shopping mall and saw the same 212 selling for much less, greedy as humans are, i bought it for 3k from that shop, the perfume was a fake, every time i would spray it the scent would make me nauseous, the smell was different and the sillage was pathetic.
I still have that bottle with me, hardly used even 40% of the scent.
Now whenever i buy a new perfume i buy from a reputed store after trying it multiple times.
I go to the store, try the perfume on and stroll around the store checking out other items, then i make the purchase if it is still appealing after 20-30 minutes; this is the case with newer perfumes which i haven't had the chance to smell.
In the case of known perfumes, i'm pretty impulsive still.
The bottle which these small perfume shops ittar shops have seem to me as discarded tester bottles or expired lot which is exported to third world countries like ours where they dilute it and sell it off.
Proper perfume makers have huge funding going into R&D; over here we have certain Majja Gamma diluting whatever amount of liquids into the concentrate to make it work. No precision, change the formula by even 1ml and you're off the hook.
Still these shops do the job in a much cheaper price.
I happen to buy an Issey Miyake concentrate from such shops once, everytime i would put it on it was make me cough and would evolve into a weird cheap scent with the passage of time. Disposed it off.