All mags are hard to close the bolt over. Mil procedure is to use the bolt hold open when loading - which automatically engages when empty, just one of those little pieces of genius Stoner made standard for combat weapons. It even allows mags to fully seat and not fall out.
If you need cheap mags, that's what you get, for range use or practice, it's nice to have them ready to go, and used enough to see which might have problems. Then the great fundamental ethics issue of AR's will present itself: are you concerned enough about your fellow human to crush bad mags, or do you sell the POS to some unsuspecting or uncaring newb?
If you're buying used mags at a gun show, you likely are the recipient of someone else's complete lack of regard. New Thermolds are likely more reliable than used GI.
Although anecdotal, someone ought to report on that. How many of which would be defective out of a dozen?