is required
I remember in my mid - late teens ( mid - late 90s) churning through tens of games at my local (independent) game store..
Typically you could buy a relatively new ps1 release second hand for 30 quid, then sell it back to them a few weeks later for 25.. in the end a not exorbitant markup and meant that as a spotty teen with no real income I could still play all the new playstation games I wanted by paying a fiver now and then, provided some person sufficiently desperate to play it had bought it on release and then sold it on once they got bored..
I gather markups in the stores have increased a fair bit since then, although as andybird mentions use of ebay means you could probably still do a a similar thing.