When I worked in a used bookstore, we'd price comics like this: Check to see what certain issues were going for at Mile High.
Then, since My Comic Shop was local, we'd check there to see what prices were going for reK certain issues in our geographic area. They were always lower for the same condition. So we'd price our stuff even less than MCS.
As a buyer, MCS is great because they grade so strictly. I'll get a raw book at a good deal, then have it signed/slabbed and it will come back a half point to a point higher than what MCS sold it for. That's nice when it happens to a mid-grade Giant Size X-Men #1 a fella wants signed by the creators of Wolverine.
Of course, as a seller, it works the other way around, too; the offer they give you for your raw books, and then what you actually get offered once they have been inspected, is usually going to be lower. As far as selling slabbed comics in their auctions, it is a gamble. I have lost bigtime on some books while other books have inexplicably brough in a nice price.
I keep going back, but am looking more towards Ebay now to sell some slabs, although I have seen the same issue there. A Destoryer Duck signed by Jack Kirby doesn't sell at $30. It gets relisted at $75, and sells. An unsigned slabbed Manera cover goes one place for $138 while on Ebay a slabbed Manara, same grade and signed by Stan Lee, goes for less than $90. Madness. Madness!
(Who are we to tell him? Well, in terms of suggestions rather than decrees, we're the people holding onto money and willing to spend it, wanting to spend it - but not throw it away, is who we are).