Those are the worst places to have rust. Depending on how bad it is you may or may not be able to fix it. Honestly, if I were to buy a used car and I found rust inside the trunk or in the engine compartment, I would avoid buying it completely.
Is it just surface rust or bubbling? If your not sure how bad it is, use some sand paper and lightly sand it, it shouldn't take long to see the metal surface if it's just surface rust. Since your car's black sand all the rust off and just touch it up with some "Tremclad" rust paint. It's black so it should cover up quite well.
I had a little rust on the inside of one of my fenders. I grinded it down and took out all the rust, touched it up with the Tremclad rust paint and the rust has never come back 2 years later.
'02 C32 AMG / '07 C280 / '07 CLS550 / '15 JKU Rubicon