Well, at least you know why pre-ban rifles bring a higher price now. They're 'grandfathered' and you don't have to worry about compliance parts. The list of compliance parts is mainly for people who are assembling a rifle from a parts kit and installing them on a ready made receiver. You should be okay buying any rifle so long as the person you are buying it from didn't assemble it themselves from a kit. The biggest worry would be if someone took a thumbhole rifle and put an imported stock back on it (a big no-no as far as compliance parts). If you're in doubt, check the importation date that should be stamped into the receiver to see if it's a pre or post 90.
So, the short answer is: Leave it as it came from the factory or make sure anything you add to it is made in the USA and you should be okay.
Ah, what they hey. I'm bored so I'll post the list of all parts that fall under 922r just in case someone wants a pistol grip or night sights on their gun. To be considered 'made in the USA' and not subject to 922r you can have no more than 10 of the following 'controlled' parts:
Receiver, Barrel, Front Trunnion, Rear Trunnion, Muzzle Attachment, Bolt, Bolt Carrier, Gas Piston/Puck, Trigger, Hammer, Disconnector, Buttstock, Forearm/Handguard, Pistol Grip, Magazine Body, Magazine Follower, Magazine Floorplate.
Any individual can assemble a non ATF controlled weapon (full auto/short barreled rifle or shotgun) for personal use so long as they do not intend to sell it. So you could
buy a parts kit, replace the trigger group and stock with american made parts and be within compliance using unmodified imported magazines.
To be 922r compliant you can have no more than 10 foreign parts from the above list, not change 10 parts so it's a little bit easier.