I also bought my 15 pro max from Facebook marketplace. As long as you are careful and you know how to spot a fake iPhone it should not need a problem. Also test the phone thoroughly before buying

Go for iPhone 15 pro or pro max. If you are going to upgrade then buy the latest with better specs.

Well I switched from Galaxy S23 ultra to my very first iPhone 15 so I went for the Pro Max. I wanted to go for just the Pro but couldn’t downgrade in the screen size department also caise at that time it wasn’t available in stock