POWER TREADS: this is the best type of boots because increase a lot your attack speed, that is one of the target of this guide.
MJOLLNIR: ok, with this your attack speed is excellent, and now you have to concentrate on your damage. This item allows you to push more fast with the passive effect, and this is always a good thing, that allows you to make a lot of money.
DEDALUS: wow, with this item sniper becomes a war machine, not only with a great attack speed and a great damage, but also with a good critical damage.
BUTTERFLY: this is a must have for a lot of agility heroes, you know, but i think this should be the last item to do in your build.
SHADOW BLADE: sniper is a very fragile hero, so this item allows you to escaper from bad situations and give you a good damage and a good attack speed, which is always a good thing.
MONKEY KING BAR: ****ing missing hero like phantom assassin or brewmaster? buy this in place of the dedalus
LINKEN'S SPHERE: ****ing nuker like lion or lina? try this
ITEM TO NOT GET: why? why do u wanna life steal on this hero? he has no life, Sniper is a sniper, he must hides himself between the trees!
sorry for my bad english :D
Sniper needs a lot of money to be an hard carry, for this reason you should upgrade the first and the third skill, that allows you to make more money. Sharpnel is good to push and Take Aim is a great skill, that give you a good vantage against the enemy heroes in early game and with this will be easy to last hit/negate creeps(push and last hit==money).
You don't need headshot and stats for the early game, but they will be very usefull in the late game (expecially headshot)
For obvious reasons, in the early/mid game sniper can be only a good "support" for the team, and for support I mean (in this case) negate creep, keep away enemy heroes in lane, mini-stun (the mini stun is very important and can be decisive to help your team in the fight).
You don't need to regret the kill "stolen" with your ulti, because money and experience is very important for you if you wanna be an hard carry :D