I'm a novice and could use some help
 

Just purchased a 90G starphire tank and a new Tom rapids pro filter with a built in protein skimmer from Craigslist. Now for the next steps, live rock and live sand. I'm planning on doing a fowlr tank so I can have a wider range of fish. I would live to buy my live rock from Craigslist as well. If I buy 100lbs of live rock from an existing tank, can I add that to the tank and be ready to add fish? Does it need to be recured? Alot of the ads say that the fish they have can come with. Also if I buy someone's live rock should I also take as much of their water as possible? Alot of questions I know can someone please help?

Just add the rock and wait for a cycle. Then add 1 fish and see if it survives lol

And dont forget to quarantine :)

If you buy rock/water/fish from someone and move everything...sure. If you want to just buy rock and take water...please, please, please...wait.
I know it's probably ok, but as a newbie in this hobby...practice (learn) patience. It WILL serve you well.