Not if they are packed correctly. Which I am going to assume they aren't.
A tight cardboard "sandwich" will restrict lateral movement in the bag. That's how I do it (& I use SA-sized bags & boards) and I think that's how everyone should pack their comics. Of course, to prove me wrong, I just got an eBay win in today, comic was in a B&B, and then slid into a photo/document mailer. That's it. Somehow, it arrived damage-free, but I consider such things to be the exception, not the rule.
Photo Mailer. Don't people know that Priority boxes and envelopes make the greatest comic reinforcements and they are free?
Well, the auction had 1st Class shipping as the only option for postage. But I can get two comics (maybe three) in between two pieces of corrugated cardboard or plastic & wrapped with bubble-wrap into a manila envelope and still keep it at or under 13 ounces. Hell, I'd have liked to see at least the cardboard "sandwich" for a $40 book. Nope, no protection other than the mailer, and it wasn't even insured.
Anyone that wants this seller's name, feel free to PM me. I am certainly not buying anything from him again.
-slym