Purchased a 6920 last week that I plan on shooting frequently. Haven't used the Colt since USMC days(15 years ago). Going to a gun show this weekend. I would like to stock up on spare parts. What would be the most common spare parts that you would want to keep on hand? Any advice for buying parts at gun shows?

Any advice for buying parts at gun shows?

First off, don't buy any spare parts for your Colt unless they are of known vintage. Don't take anyone's word for it at a gunshow-make sure they are packaged or marked properly.

You aren't going to need much in the way of spare parts for probably 10-15k rounds. If you are like me however and want to be prepared for anything then I'd start with: extractor, extractor springs/inserts, bolt, carrier, gas tube, and gas tube roll pin at a minimum.

I also have a spare fire control group, buffer spring, spring and detent kit for every AR I own-but that's just me.

Being Obsessive Compulsive is my trademark.

Depends on what kind of problems you want to be able to solve without a trip to the store.  Don't buy anything at a gunshow unless you can clearly identify it's origin as a quality part.

A complete lower parts kit, a bolt carrier group and gas tube w/pin will get you through amost any unexpected failure.  Extractor springs may last a long time, but should be changed around every 3,000 rounds to avoid problems.  It's very hard to avoid the Chineese springs, so I only buy Wolfe or original factory, unopened packages.

Magazines can be damaged and small pins and springs associated with the push pins and safety selector are easy to loose.

If I could only choose 1 thing it would be a spare bolt - one marked CMP.  That has most of the parts that wear and cause problems.  It's a lot easier to swap a bolt in the field than to change gas rings or extractor springs.