I just purchased an iPhone (on Sprint), my first one, off of Craigslist. However I am having a problem: I get home and connect it to Wi-Fi and it says I need a SIM card. I looked online a bit to see if I can buy one, but I've heard if you go to Apple you'll get into trouble with them. I took a screenshot of the Craigslist ad, just in case. I was wondering if there's a way to buy a SIM card. I think it's a used phone, but on the ad it says new but out doesn't look new, if it makes a difference. I really want to know where can I get a SIM card, how will I be able to use it, how much it is, etc.
First of all, congratulations on buying your first iPhone. :) They truly are the best phones.
Now, about your post:
There's nothing bad about buying a used iPhone, as long as it's not iCloud locked. I've done it many times; the iPhone 6s I'm typing this on is the only iPhone I've ever bought new.
The seller was right in not selling you the SIM card with the iPhone, as it's his and he just put it into whatever other phone he's using now to continue his service.
If your old phone (if you had one) was on Sprint and takes a nano-SIM card, all you need to do is stick a paper clip in the hole on the side opposite the volume buttons on your iPhone to pop the SIM card tray out and insert your old SIM. See this Apple Support article for more help.
If not, you just need to take it into the nearest Sprint store and the representative there should be able to help you. You'll need a cellular plan for it and the SIM should cost anywhere from free to $20.