I would not buy used batteries of any kind unless they were very close to free or I knew exactly what had been done to them. Right now AGM batteries are my least favorite types anyway. IMHO they would not exist except they are drop-in replacements for airplane and car starting batteries. Gels would die of overcharging in most cars and airplanes. Other than lack of tolerance for high voltage I find gels superior in every way. My 4D gel house battery lasted over a DECADE and the 4D AGM that replaced it is not doing real well at year 5. I found a gel U1 starting battery once on our recycle pile and gave it a try. This battery was 4 years old, dead, and on the trash pile. I got 6 years out of that one too after a charge
* if anyone just has
to have AGMs, group 31 AGMs are a big thing now for trucks that are not allowed to idle while parked. Sam's Club has them for something like $167 or so. They are East-Penn Dekas and are MUCH cheaper than the Worst Marine ones, which are the same with another decal stuck on.
EDIT - batteries are expensive and heavy. No one is taking them out just on a whim - they had to suck to get someone motivated to haul all that lead.