Re: would you buy used AGMs?
FWIW, I'd expect that most of the private-label batteries are just that, private label over a common product. After all, that's how battery distributors also give out warranty replacement batteries, slapping a "Honda" or whatever sticker on the same SKU that is used for their own brand name product.
BUT, private label products for large companies (Costco, Sam's, Sears, whoever) are often built custom, to a contract price, and if someone says "I need to knock two percent off that cost" it gets taken out, somewhere. So it is worth looking carefully, comparing weight and ratings, etc., when a private label product looks just like the original brand name. Even if they're different they may still be a great bargain.