I have bought AGM batteries off of Facebook marketplace with good results. Most of the vendors that service the major cell phone carrier towers replace their standard UPS batteries after several years. In most cases these batteries have seen limited use and originally were designed per commercial specs. Two I recently bought had ratings of 10-12 years at 5000 charge / discharge cycles (Hitachi CS3 HRL 1254OW FR). The guy I bought from was near Atlanta and serviced the SE. Just waited for him to travel near my area so that I could meet him and buy (shipping costs would be a killer). He performs capacity load tests on his before selling them. The two 134 AH batteries were $60 each and so far have performed very well over the last year. We used to buy similar batteries from others for many years to buy backup power supply batteries for a ham radio repeater on a remote hilltop. They will typically last several years. As any other lead acid battery, the key is to limit discharges to <50% and keep them in a reasonably cool environment. Regardless, these batteries were a significant upgrade to my previous Group 24 70 AH battery. My two cents.