That off-limits room might hide a worrisome defect
A professional home inspector is allowed access to all areas of the house -- basement, attic, crawl spaces, practically everything. Why not you? Serious sellers don't have anything to hide from potential homebuyers and will leave all doors unlocked.
Ask about any rooms that are "off limits" during your home tour, and arrange to see them later if you're interested in the house.
If a seller doesn't want you to see the attic, for instance, what's wrong up there? If the basement is locked, how can you check if the foundation is solid? And if you can't open the breaker box, how do you know the wiring is safe and up to code?
Also, look behind boxes or clusters of furniture. The sellers may just be moving, or they may be trying to hide a flaw.