It's really hard to tell much because the image 
is so small, but I don't see any glaring 
differences between the cel and the 
corresponding frame in the OVA.  If it's real, 
someone got an incredible bargain.  Even if it's 
a repro, 5000 yen isn't a bad price for such a 
nice shot.  

If you're interested, here's the url for some 
info that I put together several years ago on 
spotting AMG repros:

http://home.att.net/~jwwtlb/repros.htm

It needs some updating to cover movie and hanken 
cels better, but the OVA section still seems to 
apply.