Well, depending on the machine you might be out $30.
If it's an Intel machine (I can't look back at the moment to see if you said), you can just jump to Snow Leopard (and then, optionally, to Lion).
If it's not an Intel machine, then Leopard is as far as you can go with it (specifically, 10.5.8). If the system is working fine OTHER than this one issue, you might consider just setting up a new admin account for yourself and deleting the old one (the previous owner's). You will of course lose any stuff you've put in the old account, so you'll want to back that stuff (if any) up first.