FTF in CO w/o an FFL:
* Buyer must be 18+ years old & a CO resident (a LOOK at their DL will identify this) or active duty military with orders (the "with orders" may not apply in this situation, but I believe it does). Handgun restriction of sales to 21+ is only when going through an FFL and does NOT apply to FTF sales where no FFL is used. In CO, anyone 18+ can purchase a handgun from another individual. Of course, that's up to the seller - unless I knew you most of your life I wouldn't do it, but that's me.
* Seller must have no reason to believe the buyer is a prohibited person (no felonies, domestic violence convictions or active restraining orders/orders of protection against them). The seller is not required to know, just not have any reason to believe these exist.

Things like Bill of Sales (BOS), reciepts, copies of buyers/sellers DL, etc. are a matter of HUGE debate and would be details the buyer and seller would need to comes to terms about. Most sellers are satisfied to simply glance at the buyers DL.

Nothing else is required for anything that isn't an NFA item.