You did the best you could with the seller's photos. The value really starts to accelerate with incremental grades in the AU range. The photos just aren't clear enough to differentiate AU 50/53/55 here. It is somewhere in that range, but that's a $200+ spread. If forced to lean between mid and low AU, I'd lean toward the lower end.
Can you ask for better photos? It seems to me to be risky without clearer pics.