Do stickers on the cover make a book unpostable?

No, they don't. Price stickers, UPC labels, and Book Crossing stickers are common ones I see all the time. Now, if a kid plastered stickers all over the cover to the point that it was covering it, I wouldn't post it unless they could be removed without damage.

Whenever I get stickers on a book, even library stickers, I always take them off using some brand of 'goo gone.'  It is almost like a game for me. However, I also like making a book look better. I even use book tape to repair torn or frayed dustjackets.

"I always take them off using some brand of 'goo gone.'"

Have you ever had anyone comment afterwards about the odor? I've thought about doing that on occasion, but since my wife is pretty sensitive to most chemical smells, I've been kinda leery of sending a delabeled book to someone like that. I realize that the orangey smell that "Goo Gone" has isn't quite in the same league as the vile odor "Goof Off" has, but my wife even has problems with the orange smell.

Have you ever had anyone comment afterwards about the odor?

I find that it only smells strongly when wet.  It evaporates, and the smell goes with it.  I can't smell it at all when it's completely dry.

I am of the opinion I'd rather smell the goo gone than have the sticker.  I remove all stickers on my books as well.  I leave them on dust jackets sometimes as the paper's not always as sturdy, but on the books themselves--yeah, those things come off.

I'm not a book crossing sticker fan, either.

"Goo gone completely evaporates and leaves no odor."

Well, I've found that to be only partially correct. About two weeks ago, I had to clean some stickers off of the walls of a room our  grands lived in for a couple of years; I used the name brand "Goo Gone" in the pump spray bottle. The walls were initially painted with a good-quality Sherwin-Williams semi-gloss less than five years ago. A couple of days after removing the labels, I found that it was very obvious were those stickers were: There was a clearly visible area around the spot on the wall where each sticker had been located. Several days later, there was no change. Finally, I took a bit of 91% rubbing alcohol on a sponge and removed what appeared to be a very thin oily slick. Now, except for a few spots that look slightly cleaner than others, you cannot tell where any of those stickers were.

When I use Goo Gone, the scent can linger in the room for several days after using it!

I have a RC that says no stickers except store or price stickers as I got a book covered in kids stickers (front and back cover; the blank pages in the front and back).  I was so miffed I trashed the book after reading it because it was already well read.  I hate stickers I have this thing where when I'm reading I frequently look back at the front cover just too see if it matches what I'm reading.  And in the beginning I look at the back and read the decription often, not sure why.  I just like to be able to see clearly both the front and back cover.  I don't mind sitckers on the edge or if they are under the decription on the back cover.  Also book crossings on the inside of the cover are fine too but spralled all over the book they tick me off.    

I use a paper towel and a small amount of watered down Windex to clean book covers and a used dryer sheet to remove sticky residue.

I've tried Goo Gone, but I've found that it does smell bad, doesn't work that well and if not very careful will actually eat into the shiny coating of the book.

Textbook vender stickers...they suck.  Often large, cover relevant portions of the cover, and use permanent adhesive