It'll cost your about $75 to $150 to get started to buy tools.
Buy older iPhones, with crack screens, that aren't iCloud locked on eBay/Facebook Marketplace/Craigslist,etc.
Replace the screen.
Resell on eBay and Facebook Marketplace.
For example, buy a screen cracked iPhone 6/6S for less than $35 and you can resell them for a markup of about $50 to 75 after eBay/PayPal fees.
If you have a heating pad, which makes the initial disassembly easy, you can replace/repair in about 10 minutes.
You can watch vids on YouTube to learn how to take them apart for free. If you're not bullshytting, I'll list everything you need.
Tip: Take a lower profit and offer free shipping. Or add the cost of shipping in your price and offer "free shipping".
Tip: Restrict sales to buyers with less than 15 ratings from sellers in your ad.