"Used" has become a dirty word in modern English. Not-new cars are pre-owned; not-new mags are demo. Actually, demo is generally understood in the industry to mean lightly used, usually in good cosmetic shape as well as fully functional. If the price is right, I'm sure the mags are fine.
I have found no difference in function or quality of construction between Colts and other G.I. mags. Colts tend to be more expensive because of the name. Also, 20-rd mags cost more than 30-rd, due to scarcity. Many folks prefer the 20-rd because they are easier to use in benchrest and prone postions. Asking prices I've seen lately are $15-25 for G.I. 30-rd, $20-35 for 20-rd in good shape. Add another $5 or more for Colt. You can find mags at the lower end of these ranges if you do your shopping.
G.I. mags, Colt and others, are generally considered to be the best mags for the AR-15. The other alternative is non-metal mags: Thermold (Canadian military) and Orlite (Israeli). These are cheaper than G.I., but don't work well in all guns. They are 30-rd only. They either work or they don't, but you can't tell until you put one in your rifle. They both work fine in my Bushmaster; others report they don't fit some Bushmasters. Any other mags are a waste of time and money IMHO. There is an extensive mag FAQ at AR15.com, but that site seems to have crashed for the last week or so.
