Some folks wouldn't like that, but I can tell you I have received shipments from people here packed just as you describe, from what appear to be respected board members. Keep in mind that I tend to buy what I'd call mid-grade and others would call low-grade (VG or F) books that are generally not high priced items, so it's less a possible problem than a 9.8 Gold or Silver Age key would be.
I'd say if you wanted to be safe, state upfront what your packing style is, and possibly give options (offer higher level packaging at a higher level price).
This is what I typically do. I'll pack things in a box if the buyer wants to pay more for shipping. I'll ship things in a padded envelope if they want to go a cheaper route. Either way, I offer both. I require insurance for anything I am not willing to cover myself.
Since comics do not qualify for media mail, I use first class and/or priority mail, whichever is more economical.