The ad says its a FN mauser. looks commercial, not military to me but the pics aren't good enough to know for sure.
Personally, I think the .257 roberts is OK, but it really won't do any thing a .243 won't do.
I have a 6.5x.257 roberts that I just got, and I really wish they would have left it in 6.5 Arisaka 60 years ago when they sporterized it. The .257 Roberts is a bad cartridge to find ammo for unless you are a reloader, it's an oddball with no real purpose or niche other than being different.
It's kind of like owning a Beta videotape machine. It does pretty much the same thing that another machine does just as well or better for significantly lower cost with greatly improved logistics.