The only advice I give on this forum, is to ALWAYS COVER YOURSELF FIRST. (CYA)
Don't put yourself in a situation that could possibly risk your business, profits, RIGHTS or freedom because you want to make a quick buck, or you want to help a buddy out. Even if you keep a separate bound book for your personal collection, remember that you are in this business to make a profit, and to keep yourself LEGAL at all times. Don't ever let anyone convince you to do a personal transaction (FTF) on an item that is in your A&D book, it isn't worth the few bucks you might potentially make.
All of that aside, I log ALL incoming weapons into my A&D book regardless of if I want them for my personal collection. If I REALLY want them, then I do a 4473 on myself, but I don't sell my personal weapons anyway.
If you find that you buy and sell alot of items for your personal collection, then you might as well leave them on the A&D book so you don't have to worry about the FTF problem in the future. Cover yourself and do a 4473 on all buyers, and if they don't want to do a 4473, then move on to the next buyer. There is ALWAYS a legal, willing buyer. Don't let a customer convince you that they have the cash now, or whatever they offer is worth it, you doing a favor for them could potentially lead to problems for you.
Again, just make sure you cover your bases, and make sure you are legal at all times.
Enhance your personal collection all you want, but remember that you are in the business of selling weapons legally, and it really isn't hard to stay that way.