My take...
I don't think you're missing anything. Your SGC 40 Ganley looks nicer than the one on eBay. The eBay Ganley BIN is just way offf. It's a low/mid grade common that maybe is somewhat tougher than average but is nowhere near deserving of the $600 price tag. Unfortunately, it's not the only far out BIN both on eBay and other online stores.
Having said that, there are some good BINs on eBay and elsewhere if you look around. I have watchlisted/bookmarked some T206s that are graded in the SGC 20-40 range that look much better than their grades and are priced at $20-40.
I think about it this way - I could pick up at least 15 of those nice SGC 20-40's for what I'd pay for that one overpriced Ganley! It's not a hard choice when put like that.
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