Worth Buying Used AGM Batteries?

I'm quite familiar with the use case. Cell towers, for instance. They are meticulously maintained, however, i do know that they go bad even without cycling, eventually. I hear about banks not performing to spec all the time -- but when you have 80 banks, that tends to happen.

Essentially you've bought a bank with a max capacity of 11kWh. You don't want to cycle that past 50% if it can be avoided, so for regular use, it's less than 5kWh.
For $400, that's a pretty good price, assuming you really get that performance. The cost per cycled wH will still be higher than LI, but since you're using this for emergencies, that doesn't really come into play. Having said that, most people start out that way -- then they inevitably go off-grid.

For the off-grid use case, there is almost no excuse to use LA batts anymore. Save up to buy LI - you'll end up spending far less in the long run.

So TLDR, i'd say you got a decent deal for your use case. Lots of variables for their condition, so hopefully that will work out.

My thoughts exactly. They'll die due to calendar life before they die from cycle life and even LI has a calender life.

Assuming they still have full capacity, that's a great price for energy storage.  At ~114Ah each rated, that gives you ~11kwh of batteries for $400, or a cost of $36/kwh.  I thought I did real good with LiFePo4 at ~$100/kwh (but to be honest, those Tenergy 32650-cell LiFePo4s now at at least 5 years old, show absolutely no signs of degredation nor have they needed any maintenance). 

Most commercially available LiFePo4 setups are in the $300-600/kwh range, making them quite pricey.  So $36/kwh is real cheap...assuming they have the full capacity.

However, just because they're kept at float doesn't necessarily mean they're in great condition.  I'm not the best lead-acid expert--or even fit to be called a lead-acid expert.  What I know is that a set of 8 Datasafe 12HX505 batteries, never deeply cycled to my knowledge (though that doesn't mean they couldn't have been cycled a few times) ended up after a couple of years with only about 30Ah of capacity.  Sure, I am aware they were gassing a bit when connected to a solar system for a couple years (TS-MPPT-60 settings I guess!), but whatever happened, their capacity was basically nothing after about 5 years.

That is quite the deal. But i guess that's what you can manage with a 32650 wall.

My configuration was about 240$/kwh. The biggest issue was getting the damn things shipped here, being a hazmat item.
So far i have been extremely pleased with the performance, given the pricing and origin. They have lived up to their specs and no signs of degradation yet.

Ya there's definitely a risk with used batteries, especially when it comes to lead acid. Only time will tell with my batteries.