If the rifle offered is indeed a Ruger semi-auto in 7.62 mm Nato and looks liked an upscaled Mini-14, buy it now. There's only about twenty Ruger X-GI made.
A prototype to the XGI always threw the fifth shot and the engineers' solution was to make the slide (op rod) heavier. Bill Ruger didn't like that solution but twenty were made and the guns were sent to gun rag writers. The decision came down whether to make them or not. It would cost $1,000,000 to re-tool the production line to make them. Bill Ruger thought it wasn't worth the money as he could make $1,000,000 just by keeping the Mini-14 in production. Thus the X-GI project was shelved.
BTW, the receiver was about a foot long. That's substantially longer than either the M-1 Garand and the M-14. When I visited Pine Tree Casting, I saw the wax casting of it and pulled it off the shelf to examine it and was shocked by its length. Of course, I had to put it back and one restriction was that we were not permitted to take photographs in the factory.