Just have to trust the seller ratings and most will typically show an image that includes the serial # which you can look up for more information. I'm very picky about who I buy from and try to wait till I find a "mint" listing where it's almost new. Sure, always a risk, but even when I buy new I don't like buying directly from Apple anymore... I typically go through B&H to avoid the $300-400 tax hit on top of the $3000-4000 price tag. Apple is expensive but add the tax plus Apple care and you're almost another grand deep lol. I also took video of me opening the package when it arrived, including inspecting it, cleaning the outside before opening it to inspect the screen, etc. Just in case there was an immediate issue I could see.
I don't think I've purchased a new Mac since 2007 when I got the first Mac Pro. Otherwise I've gotten them all from Ebay including an 8-core 2009 Mac Pro, MBP 2007/09/11/12, MBA 2013, MBP 2015, and now this one.
