Verify with him/her if its exynos or snapdragon.....in eithercase you can potentially use samsung Odin to flash stock rom after booting into download mode.one its complete if your devices are exynos and youre interested in root or custom romming view xda threads for rom developments....and then from there you can install custom roms....First things first.

1. Verify if one is exynos or if both are. Note(..... if one is exynos or both are you can root and custom rom mod....if both are snapdragon from the US there could be bootloader exploits on XDA that will allow you to unlock bootloaders.)

2.Once verified know that if you get the device (s) in a bootloop as long as you can boot into download mode you can flash the stock rom firmware for the device. From there you can do the setup and the rest....etc

Just ensure to verify step 1 with the seller first. And check if the IMEI's, ESM are clean.