With a VERY few exceptions, any M14 rifle made by the government Springfield Armory*, IHC, TRW, or Winchester was made as a selective fire weapon and is a machinegun under the law, whether the full auto parts or the place to put them are present or not.
Yes, I know all about one court case involving TWO rifles, which is why I said "exceptions", but believe me, those guns would not be for sale at $1200. Any M14 rifles loaned by the government to state associations remain U.S. property; one of those for sale is not only illegal as a machinegun, it is stolen property as well.
*Springfield Armory, Inc., a private company, has made thousands of legal semi-auto near-copies of the M14, called the M1A; other companies have also made semi-auto only copies. One company made M14 rifles by welding scrapped M14 parts together; ATF seized their products and arrested the owners. The owners were convicted of other charges, but the rifles are contraband nonetheless.
Jim