There are those who would say that 300 bucks sounds excessive, given what larger horizontal mills don;t fetch at auctions. OTOH, you are buying the mill from a private seller who has something tied up in it. He is not in the same position as a larger business who may actually profit in the end by selling the machine at a loss. It is a solid looking little mill, doesn't look abused, and has a vertical head. It appears all intact including power feed to the table. It is one of those little machines that is small enough to go into a home/basement shop & meets your requirments. These things don;t come along with any certain regularity, so I would grab it. I would say 300 dollars sounds like a fair price.
I am guessing the spindle taper may be a Brown & Sharpe number 7 or number 9 taper. A number of the smaller milling machines from the 30's-60's used the B & S tapers.
I am also guessing the Jefferson Milling Machine may have been a WWII era product. There were a few smaller milling machines that came on the scene for WWII defense work and ceased to exist right after WWII. Most of those smaller mils were built for production milling operations. They often had hand-lever and rack feeds on the X-axis for production work. The Jefferson Milling machine is built as a regular little knee-type horizontal mill with screw feeds to all axis- making it an interesting machine. It is all intact, and from the posted pictures, nothing looks to be busted/brazed or missing. I would not hold out much hope for locating Jefferson let alone any parts, so it is good the machine is all there and intact. The fact the seller has used it for making telescope parts means that he was likely able to get close work out of that little machine. It should be a good little milling machine.