How in depth do you look into a comic book back issue when you purchase it from a shop?
If I'm buying a book to fill a hole in my collection and the covers look good and it's about $20 or less, I pretty much just go for it.
If I'm buying an older book with greater value, I'll dig into it a bit more. Take a good look at the interior, and decide if my opinion of the grade is commiserate with the shop's opinion.