Depending on the version of OSX in question, the system may need all the
RAM it could handle and maybe a larger hard drive, too. Sometimes, it is
easier and better to include the equivalent of a retail boot disc and some
basic software and let whoever gets a computer figure it out.
That may mean knowing the minimal RAM requirements and
other specifications for the system and the OS you've got.
And making sure the configuration can handle these properly.
Things like Firmware Updates and how much a model can be upgraded; plus
information on configurations you may see and what they are, can be better
understood if you get a free use download of MacTracker from their web site:
http://www.mactracker.ca
Information and knowledge with the wisdom of when to do what, or leave
well enough alone, comes from experience and reading. Hope you don't
give up, older Macs are better than being dumped in a recycling bin.