If the seller has a good rating, and has ample photos and a good summary of the device (ie, detail of any scratches, cracks, previous dips in water, drops, length of use, etc) then go for it. As for the battery concerns, it has a 3000 maH battery that can get you through the day on one charge before having to charge again. Some phones have batteries now that are much more capacity than that, but it does support fast charging, which can get ya from 0 to 50% in roughly 30 minutes, and zero to full in ~ 1hr 25min. Also can charge wirelessly and supports wireless fast charge. Wireless fast charge isn t quite as fast as adaptive fast charge through the cable and power brick, but it still charges at a good rate. Good backup plan in case the usb port were to fail for any reason.
With moderate usage today, I got 19 hours before plugging in, and I still had 20% then. Over 5 hrs screen on time, which is good. So I think battery life won't be a problem for you. And coming from an s4 and s5, it's definitely worth the upgrade, and the difference will be quite noticeable. Does support microSD up to 200GB. So while the 32 GB internal storeage may seem paltry, that external storeage support can significantly expand your room for photos, media, etc. If those phones are in good condition, you are getting a good buy.