That's ridiculous. I haven't had a return for grading in at least 5 years and the one return I took the buyer agreed it was NM he just didn't like it. It does happen, but only if you don't do your homework.
Grade the book well and scan the book well the rest will take care of itself.
Sure there are cons on ebay, but the % goes way up when you are selling slabs I think. If you look in the ebay threads a larger % is involving a slab for all types of reasons.
It is impossible to grade a book well when at any time someone can disagree with your grade. Your grade could be spot on but they still disagree with it. I know one LCS that when you sell them books, they are a 3.0, when they go to sell em, they're 6.5's.
Just ONE?!?!?!
When those types of LCS are firm with me on their selling price and say, "That's what it's selling for on eBay!" I reply, "Yeah, but this ain't eBay. This is I'm standing here with cash bay!"
Which is better because I'm looking right at it as I make my offer, so that there is no return/you can't grade/my mailman's dog ate it issue. LCS's need to value that, because eBay can be a Wild West for a seller. Easier to control who you buy from than it is to control who buys from you.