I've been a licensed C&R for 20+ years, know a lot of fellow collectors, and haven't heard of anybody getting an office or a field audit. I have heard internet rumors of audits, but none have ever been verified when push comes to shove...just "I have a cousin who knows a guy who...".
If your collecting is a business, you become a dealer and need an 01 ffl...harder to get, random inspections not uncommon and sometimes not requested in advance.
You CAN sell rifles from your collection, but only on the basis of improving your collection. For instance, no problem buying a 3-fer or 5-fer, picking the best one or two, and selling the rest. How often you do this is a gray area. E.g., if you have a table at every gunshow, someone might be interested in whether you're collecting or making money. Up until the time I moved to the PRK, I sold a few maybe 5 or 6 times a year, ATF in Dallas said no big deal.
Get the license!
-hanko