I'm not a fan of Windows 8 myself although there is one small tool you can install that will certainly help any future user - if you install Start8 from Stardock software onto Windows 8 then you can use it very much like Windows 7, but it also has the Metro interface still there.
I got Windows 8 when it first come out and with the Start8 software I found it works fine, and I have also installed Start8 on a few other computers too with Windows 8 where the owners were struggling and they were also surprised about the improvement it makes - the main thing it does is just puts the start menu back where you want it.
Also it doesn't really matter whether or not you put 8 or 7 on it as Microsoft are bringing out Windows 10 and I've heard that Windows 10 is going to be a free upgrade for all Windows 7 and 8 users - http://www.pcworld.com/article/2873214/windows-10-will-be-a-free-upgrade-for-windows-7-and-8-users.html
So either way the new owners will be able to upgrade to Windows 10 anyway.
As the laptop originally had 8 on there I'd be strongly inclined to put 8 back on there and sell it with that.
Also even if you don't have the recovery CD the licencing system on 8 is very different to the past - firstly you don't have a COA - if you look on the base of the laptop you'll have a strange Windows hologram sticker with no COA - this is proof that Windows 8 is licensed for that computer, the OEM code is all built into the laptop already, so if you just get any Windows 8 disc that is the right version and install it, it should be able to figure it out. If you don't have the installation media there are a number of ways to obtain it - you can go to the original laptop manufacturer and they should be able to supply it (might charge a couple of quid postage), or if you look online I'm quite sure Microsoft offer it online somewhere.