Re: bought vs had bought
EDIT: Oops. I didn't see page 2 when I wrote this. Fighting Spirit is completely right!
Actually, in your example the buying and the paying took place at the same time. There is no event before another event.
The past perfect is really only needed when you can't tell which came first. In your last example, you can't tell if the cleaning was done before the arrival if you just say "I finished cleaning when he came." It actually sounds like he came and then you finished.
When you have markers like before, after, since, etc. that make it clear what the order was, you (generally) don't need the past perfect.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.