It might be cheaper to buy parts kits in bulk and resell them vs buying functioning firearms and breaking them down for parts.
I can understand if you bought a gun, rebuilt it for a different purpose, and sold the parts you replaced parts to offset your cost (ie, you found a great buy on a:
1) lightweight Remington 700 with a wood stock, and you rebarrel it with a 308 heavy barrel, put a synthetic stock on it, replace the trigger guard with a steel one, install a detachable magazine system, and replace the trigger with an adjustable one.
2) a police turn in Glock which you complete rebuild, including a threaded barrel, night sights, and extended controls
3) a Mossberg 500 with wood furniture and a 28" barrel with a complete set of choke tubes, and convert it to an 18", synthetic furniture, ""tactical" shotgun.
In any of the above scenarios, you'd have a couple of hundred dollars worth of parts left over from the projects, which makes explaining the expenses to your better half!