I saw those on my local Craigslist too and was also skeptical.
I had thought about getting some of those long-term clothing-storage ziplock bags where you are supposed to vacuum out the air, since they seem to be air-tight, but they do not come cheap, and I'd need different sizes to handle my collection, but some of these are made to hang on a hanger in the closet, and also to defend clothing against moths, so if those are also air-tight, then maybe that is an option as well...
If I had the cash, I'd take a page from kohanmike's solution, and either build a hermetically-sealed cabinet, or convert a closet into an actual real humidor, with fully automatic climate control, but the prospect of relocating soon has put this off into the next home where I will live.