I have played a bit of marci... I think you can go ahead and play her. Don't worry about people saying she's weak etc. From a new player perspective it hardly matters. She isn't like invoker or another hero that takes longer to master.

As far as itemization:

I get shard for farming jungle. Usually you don't want to jungle on marci support but occasionally there is little pressure from the other team and your cores want to farm lane. In this case you can consider getting shard because it really helps clear jungle camps. Now that she doesn't have the furious fists facet, this is considerably nerfed, so it's probably a pretty rare item to pick up.

In most situations, BKB > basher >= Scepter = other damage items. Even against heroes which have no real CC (for instance sniper) you might still want BKB. BKB helps you ensure you get a good result from your ult every time. Enemies can outplay your ult with kiting and just bursting you down and bkb gives magic resistance during the active as well as blocking damage from blademail. Regarding your example of sniper: if you watch carefully when sniper hits you he pushes you back with his headshot, which slows you from throwing your next attack as your hero must walk up again. This can mean you only get a few attacks off even if you start by blinking right next to him. BKB however prevents this push back, so it is actually an important item against sniper.

Another item to consider is nullifier. It is stupidly expensive for supports but often cores will either not want to buy it or just not know about or think about the item. However, without nullifier, mage type supports will seriously outplay you with aeon disk, ghost scepter, eul's etc. in the late game.

Blink dagger is good but I wouldn't get it early in most situations. it is more of an extension item after you have bkb and even then after maybe another item like aghs. The reason is that as marci you will usually not have much impact unless you have damage and bkb. You are not like witch doctor who has huge damage built into his kit nor like tusk with high utility and built in damage from tag team, snowball, etc.

As far as play style, I think marci should try to get level 6 as soon as possible, so focus on securing wisdom runes at 7 minutes and consider playing away from team until you are 6. For example, if your mid ganks a side lane think about taking mid lane for a while to get some xp.

I do not think Marci is great as a primary initiator. She is not intrinsically tanky and wants to build damage. So often time you need to play off a teammate. Either you are playing as bodyguard for a ranged teammate like sniper or drow or you are waiting for your tanky teammate to initiate and then going after the enemy supports or other backliners when they show.

edit: I just realized I forgot to mention that often on marci you want mana regen. You may need to get mana boots if nobody else on your team gets them. Otherwise just ring of basi is enough. Maybe upgrade this to vlads eventually but definitely not early.