Sniper is a hard carry, and hard carries are very hard to build up. His sets of skills are just plain normal compared to other heroes but his late game what makes sniper very dangerous. To be able to dominate the game, sniper needs good farming. Good farming makes good items and good items makes sniper shines at late game. The skills of sniper will help you farm more gold. Though you are a hard carry, you should invest for early survivability items. This will help you to stay longer in team fights or in any lane. His late game presence with correct items, sniper will go to being squishy to a hard carry.
Starting DOTA 2 Items for Sniper
You need regeneration items for sniper such as healing salve and tango. Remember sniper is very squishy hero and squishy heroes tend to have a low hp. So you need to watch your HP and always at full health. Slippers of Agility are for early base damage and Iron branch for mixed little benefits. Any of that four items can be purchased in any quantity meaning you need to decide how many of Healing Salve, Iron branch, Slippers of Agility or tango based on your enemy lane.
Core DOTA 2 Items for Sniper
Get phase boots first for movement speed and bonus attack damage. Sniper don't have an escape skill so you need atleast an instant fast movement speed. You can choose in any order for Ring of Aquila and Magic Wand based on your enemy and your needs. If you need more survivability, go for Magic Wand. If you need mana regeneration go for Ring of Aquila. You have already Iron Branch and Slippers of Agility so go for those two items. Mask of Madness is your lifesteal plus good output damage and more proc critical.
Luxury DOTA 2 Items for Sniper
Get Monkey King Bar and Manta Style but you need to decide either Monkey King or Manta Style you will get first. If your team is dominant, get Monkey King Bar otherwise go for Manta Style. Manta Style gives you survivability and boost your total output damage. Monkey King Bar gives you an insane bonus damage and a mini bash. Daedalus is for critical bonus damage and since you have an Attackspeed, Mini Bash and High bonus damage, Daedalus is very good items for Sniper.