Take it out of the scratched up case. Then smell it and feel it. Does it feel antique or modern?
To me, the card it too centered, the borders are too clean, and the edges look untouched. It would be like a PSA 6. So would the guy you're buying it from have a $20K card? I would be very worried. Make a deal that you send it to PSA/BGS and if it gets an AUTH or number grade, you will pay him $2K or whatever. Otherwise you get a full refund, but you eat the grading fee.
Addition: doesn't seem to have the pixelation I would expect from an original 1951 Bowman. Look at this one to compare. Also, it seems like Mantle has a sunburn. Are you in Dallas right now? Could you drop by Beckett with the guy and get it validated in person?
1951 Bowman - [Base] #253 - Mickey Mantle [BVG*2]
Courtesy of COMC.com