I find the strength of sniper is that how you respond to your enemies picks allows you to almost always win (the exception being when there are 2 invis gankers on the other team. The main thing you need to make sure that happens is that you don't lose the upper-hand when it comes to early gold and levels, sniper is COMPLETELY useless before lvl 6.
The reason I always start with a wraith band is because it makes last hitting and denying much easier while the enemy is still less likely to try a gank. Next you have to choose between TP or tangoes:
Get the TP scroll if you think you'll need to escape or assist a gank if the enemy ends up behind a friendly tower. I'll normally get tangoes if I end up against a bad mid lane opponent either with a stun Tidehunter/Sven or quick initiate like Luna.
Your first mid-game item will also depend on the other team:
- a shadow blade to escape if you're going to be harassed early by invis hereos ~lvl 2-4 (don't get this if you don't need it, you want your Morbid Mask quickly if you've skipped tangoes/potion)
- power treads are a great second item if you're not going to push and also don't need to hide in the jungle
- a mask of madness if you're going to be harassed a little later lvl 5-7 (it basically means you can run away faster, jungle easily, permabash enemies trying to get away and bring down towers by yourself pretty easily, just remember the enemy team IS coming to get you when they see alone, so keep half your mask of madness spell duration to run unless you can see the whole team.
- a ring of basilius to finish your ring of aquila with your wraith band (get it before you think theres going to be a team fight. It will give your team more armor and give you more mana for assassinate.
- a monkey king bar is the compliment to your mask of madness giving the damage while the MoM gives the hits/second. Once you have this you can harass the enemy a bit then use MoM to kill them rather than using MoM to harass.
Late-game items should basically be anything that gives reasonable DPS such as:
butterfly - good for avoiding damage if you're getting targeted hard
daedalus - great for team fights (you will land some BIG hits)
boots of travel - very good for evading the enemy and also for pulling them away from your base by attacking theirs
desolator - great for a taking towers if you decided pushing early was more important than MoM
mjollnir - you should only get this if you get desolator earlier or you're trading in MoM super-late game once you've run out of slots
A good situational item I sometimes go for is boots of travel after a tier 3 is gone if our team gets killed before we get barracks' (I know it feels like a big waste of money because you already have power treads but you'll find you can die inside their base and get back there before they push yours which is a real problem when they end up half way to you and the rest of your team is waiting to respawn. They will have to turn around for or you will decimate their base.
Don't pair a desolator with a MoM the two unique attributes don't stack and you won't get armor reduction on targets. If you stayed in your lane to go straight for desolator and pushed rather then get MoM, you're better off getting a mjollnir (starting with the hyperstone).
The reason I get Headshot first is to get easy last hits/denies/stuns which are super-important because sniper is so gold dependent.
The reason I get Take Aim next is because you're not likely to get threatened until a little bit later.
The reason I don't get Shrapnel until late-game is because it gives less control over last kills, pushs towers when you want to stay close to yours anyway but mostly because it eats your mana denying opportunities with assassinate.
I only give Shrapnel 1 level to:
- give vision of tier 3 towers so I can attack them without being attacked by them
- reveal heroes on high ground waiting to kill me
- slow enemies chasing my team, don't make the mistake of trying to slow enemies chasing you with it, it takes more time to cast it then the chaser/s will be slowed by
Assassinate is levelled 1,6 and 11, no suprise there, although you can delay this if you're on low mana and you think you might need to use it sometime soon.
The reason MoM is good is because it gives you quick power at a much lower price then mjollnir, you'll never have over 1000 gold on you in case you die (because morbid mask and the recipe both cost less than that) and you can get out of lane and jungle when there's a threat.
The reason MKB is so good for this build is because it means you won't be missing the tier 3 tower while you attack it out of range when it's given visibility from your shrapnel. Pairing it with boots of travel is situational but can be the icing on the cake.
If there are squishy initiators like luna on your team it helps to get the shadow blade so you can mini-stun her chasers before disappearing yourself, if you have to choose let others die, no one needs gold to win as much as sniper.
I know people are going to rage on me but don't be scared to steal kills using assassinate if it means getting your first big damage item i.e. MKB/butterfly/desolator/daedalus (until you get one of those items your hits/second have no real effect on enemy team).
It helps to use assassinate to initiate in certain situations, but mostly it will just mean their team will flee before yours rather than the other way around (you don't and shouldn't be standing still using assassinate when their team is rushing towards you anyway. It can mean the difference between your team losing that fight or their team splitting up. You should be able to chase very well with MoM anyway so don't see it as too much of a loss (of course if you're in a 1v1 feel free to save it).
Never get a divine rapier as sniper, if you're going to win it will happen anyway, you're better off holding onto the gold or trading in your MoM for a mjollnir. Heck why not get a blink dagger for even better chasing?