You can do a good deal of identification by going to USCARBINECAL30M1.com and select components. Many of the pro's here on the forum don't trust that site a great deal, but it's interesting and handy for identification of most small parts. Navigate around that site and you may find answers to most of your questions.
Generally, you have an imported mix-master arsenal rebuild. Imported by Blue Sky. Not saying that's a bad thing and hoping you have a good shooter on your hands. Good shooters generally go from $700-$800. Some of the experienced members will drift by and ID most of your components, meantime...spend some time on the site I mentioned.
I go along with Americanboy on the value estimate. The RIA mark (Rock Island Arsenal) suggests your Saginaw, Grand Rapids (S'G') carbine was rebuilt after the war and was then loaned/given to a Military Assistance Program recipient, probably S. Korea and imported back to the U.S. by Blue Sky Productions during the mid 1980's. It has a mix of various manufacturer's parts, Inland hand guard, Inland hammer and a PI marked rear sight first used on Inland and later used for post war upgrades. The Underwood barrel is most likely original to the carbine. Not sure about the U mark in the sling well. Could you post a picture?