I bought 3 days ago MIM HSS strat, i have fallen in love with it except that damn low E string that keeps buzzing only on open.. .when i fret it nothing happens... somehow buzz seems to go away if i press the string on top of nut ( a bit , sustain is same ), so wtf is wrong ? When i got guitar it was tuned in D# maybe truss rod needs to be adjusted?

Sounds like it might be a truss rod rattle.

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OK, I'll hum a little bit, got it now?

Your nut slot is either too low, OR, and most likely, you do not have enough wraps of string around the tuning post, and this keeps your break angle of the string over the nut too shallow. Try restringing with a fresh set of strings, and this time wrap about 3-4 winds at least around the post BEFORE you tune to pitch.

I didn't understand the part about strings , they were strung in factory as far as a i remember and they got like 3 times if i recall correctly wrapped around that tuner...

As long as the last wrap around isn't right at the top of the tuner post you're ok.
Avoid having too many wraps, particularly on bass strings. It just leads to tuning instability.

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No no and no

I counted now it got 2 wraps and i checked other guides how to string guitars and looked at this , it got properly strung etc... except i still got this buzzing... im out of amp as i had to sell it to buy this strat ( it was on new year sale they cut the price for 30% ) so basically im playing it without amp... i heard by some people that it is ok to buzz , others say it needs set up... i asked salesman in shop when i was buying it does it need some set up he said only if i need to change other gauge of strings... everything was set up in factory... o.O buzz doesn't happen when lightly strike the string, but my playing style is that i got used to strike medium/ hard strings ( though they never rattle ) another thing is i found out the lower the tune is the more it buzzes...

im sorry for making stupid questions, but can this buzzing cause a big problem to a guitar ? i noticed that i make hard strokes on bass strings unlike on treble... for example on high e string i apply almost no force , while on low E string my hand strikes strings kind of hard ( listen to wicker man by iron maiden ), or i should take my guitar for set up... i got used at nights when everyone sleeps to play this strat without amp just like on a acoustic guitar

i forgot to mention when i apply stronger strokes on other guitar strings none of the actually buzzes...

Well that will do it, because the first fret sits a touch higher than all the rest. It still sounds like your nut is cut too low for your technique.
It's a new nut or adjust technique I'm afraid.
If the guitar is new, you could try taking it back to the shop. If not a new nut will cost you around £40 fitted, in the UK.
So try hitting the strings a little softer eh?

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No no and no