as long it's from a reputable seller like perfumeberry, fragankita, acharr, fiqeera, luxuryperfume
should be legit

buying online is usually cheaper than buying from physical store.

is you can wait 2weeks-1month u can buy from fragrancex

for physical store, i think the most are about the same price. in klang valley u can try the perfume shop at melawati mall. not the cheapest tho.

montblac explorer cost rm400 for 100ml

malaysia online perfume store sell about 3++

fragrancex u can get for rm200 only 

If possible store in dark & dry cupboard away from aircond, the worst is bathroom
most of it from the official distributor, like luxasia, prestige but their l'oreal brands idk from l'oreal malaysia or overseas
If total less than RM500 won't kena tax & if ure in kl/Selangor no warning letter from pharma malaysia, other states Idk