Poly-bags will not destroy your comics. We do not preach fire and brimstone. The fact is, polypropylene and polyethylene are stable enough for the short term storage of collectibles.
If I were a dealer or collector and wanted to keep oily fingers off my wares and valued collection, I would definitely put my "For Sale" comics into poly-bags, but only for a short term, say a couple of months. But if I wanted them to look their best, I might try Mylar. No comic looks better than one displayed in a Mylar 4 mil Sleeve. I would think the extra minimal cost is worth the expense to perhaps get a better price in the sale.
Taking this a little further; won't the customer believe, and rightly so, that I am treating these comics with the best care possible? On second thought, if I were a dealer or collector, I would put every comic into Mylar.
Poly-bags will not actually destroy your comics. They will simply afford a lot less protection against the elements.
How does Mylar, which is biaxially extruded polyester film, afford more protection? The poly-bags that most everyone sells are "blown" films, affording very low density and dimensional stability. Mylar is an extruded film that is simultaneously stretched in two directions to give it maximum strength. In fact, it resists penetration by gases, such as oxygen, 300 TIMES more than the poly-bags. As for strength and stability: it is also several hundred times stronger and more stable. Keep in mind, that we are not talking about 4 or 5 times better as compared to their price; we are talking about offering the kind of protection for 100 years which poly-bags cannot offer for even one year.
Are the Mylars better to use than the cheaper poly-bags? Poly-bags are acceptable for short term storage while the comics are for sale as noted earlier. But if the storage period is longer and you do not switch to Mylar, then you qualify as DIRECTLY proportional.
Advantages of Mylar® Type D Compared to Commonly Used Plastics:
Resistance to diffusion of gases like oxygen, Carbon Dioxide, Sulfur Dioxide, etc. is 350 times greater than polyethylene.
Permanence. There is no noticeable change in storage-100 years when compared to 2 to 5 years for others.
Resistance to moisture, insect attack, fungus, mold, mildew, acid, oils, grease, and solvents is "excellent" compared to "fair" for others.
Strength and creep resistance is 10 times that of polyethylene, one-third the strength of steel.
Volatile Additions--Mylar® contains no dangerous plasticizers, slip additives, surface coatings, antioxidants, acid-hydrolysis compared to other commonly used plastics.
Acid-Free Cardboard vs. Ordinary Cardboard:
If you use backing boards, use only alkaline buffered acid-free board. Be sure this specification is in writing and from someone you trust. There is no use stiffening your comics and saving a few pennies if the boards' natural acid turns your comics into "stiffs".
If you do use poly-bags for storing your comics, (after being shot for crimes against comics), then you should most definitely use AF boxes. If you already store your comics in Mylar then the Acid-Free nature of the boxes is not essential, but it is still desirable.