For what it's worth, we've been using deep cycle batteries in our solar for about 10 years. Getting decomissioned batteries is risky because how were they stored when decomissioned? If they are stored for a long period that's not good.
If they weren't completely charged when they were stored, they will probably not be good for any length of time. How those batteries are used during their life cycle is crucial. How they were treated for the first few months of their cycling can affect years of performance if it wasn't done right.
If they were routinely taken down below half that lowers their life span. If they were not charged completely back up every day, that shortens their lifespan, and that's easy to do in the short winter days.
All deep cycle batteries only have so many cycles, and if they are almost at the end of their life span, they aren't worth it. It's often a real struggle swapping them out, and to do that every year or two just because you want cheap batteries is exhausting. The older, lower ones will bring all the others down to their level, and you may not even realize it. that's why you're supposed to get the same age batteries all at once.
If the voltages are too low because they are failing or they aren't getting charged up enough each day, they can burn out refrigerator motors, and other appliances. Even if they are DC appliances.
And where do the used ones go? They are extremely heavy, and if you're having to deal with them in the mud and the rain, it's an awful job. You need extra floor support underneath them. They're not supposed to be in the same room with the controller and the inverter.
It's hard to buy really good solar batteries because they are extremely expensive. But you will have nothing but headaches if go cheap. Never knowing if they are performing enough to supply what you need, especially in the short days of winter and early spring is exhausting. Going solar is not cheaper. Where I am it's more reliable because the power goes out a lot, and there's only two of us. But when things go wrong with it, everything else stops until it's fixed, and that usually means a pretty good sized infusion of $$$.
"Life flows on within you and without you"...George Harrison
Coastal California, USA, Mediterranean climate - no summer rain, a little frost mid-winter