I don't see a problem with it. You're paying a bit extra for the convenience of not having to wait 4-6 months for it to arrive, as well as for knowing exactly what you'll get ahead of time. For the bargain hunter, or the person who just wants one Garand as a representative piece, the CMP is definitely the better choice. But it's a crap shoot as far as what you'll receive, and when you'll receive it. For many collectors, being able to sift through WTS ads to get exactly the one that you want (whether it's a certain serial number range, the way the stock looks, the number of correct parts, the bore erosion numbers, etc.) is worth paying slightly more.
Apparently, other people are okay with paying a premium for those perks too, as evidenced by the speed at which these $800 Garands sell.
Besides, I don't think these $800-$900 Garands are necessarily overpriced. I think the CMP is underpriced.