I don't know a lot about ar's except what I've read on line.I came across a colt ar-15 that has the 3 way switch on the receiver ..safe...fire...and auto.the switch will go into all 3 positions.The dealer[a friend]says that it is a semi auto only.What kind of a receiver is this?Dealer said it is a pre ban.Is this some kind of priemum receiver.Is it worth buying?It's got a heavy colt bbl and he wants 800. for it.

if it's got a hole above the selector on both sides, and he isn't transferring it on a form4, run far and fast.

I don't understand your answer,as I said,I don't know much about ar's.

Notice the pin over the selector hole(large black cylindrical object over the pistol grip).  The pin indicates that the firearm has an auto sear(visible in pic), or can be equipped w/ one.  If the gun has an auto sear, or has a hole for an auto sear, it is going to be classified as a Machine gun.  Machine guns transfer on a form 4, since new manufacture of machine guns on form 1s has been prohibited by civilians since 5/19/1986.  If the gun(if it were indeed a machine gun) the dealer is trying to sell you is not going to be transferred to you on a form 4 then it is not legit, hence my "run" comments.  Also if it were indeed a true MG lower, $800 would be the deal of a lifetime on it, if it were transferrable.

My advice would be to open the rifle(seperate upper and lower half) and inspect the FCG to insure that it was legal, just to be safe.  If you find it is legal, $800 seems like a good deal if it is all colt.