My guess is that it will remain available. At least in beta, macOS Sierra system requirements exclude several models that will run OS X El Capitan. That's a different situation than Mavericks/Yosemite no longer being available, since any hardware that runs Mavericks or Yosemite will also run El Capitan.I hope El Capitan will stay in the App Store.
You can, as alluded to by others, create a bootable USB installer of El Capitan and provide it with the computer you sell. The installer will not be linked to your Apple ID.