There is a lot more used APC stuff out there which keeps costs down. The APC units (at least until you get up into large datacenter units) are definitely built to a price point, though. Adding features like the LCD front panel and keeping the price the same means that the charging circuitry isn't as good as it used to be. The newer APC units are essentially unrepairable out of warranty. The older units were less complicated and you could trace though things yourself to fix them if you had the skill, though I value my time enough that fixing a 1500VA UPS vs. buying a replacement used unit is a no-brainer - just buy a replacement. APC will not provide any support for customers wanting to repair these units. I find the "sweet spot" for those smaller Smart-UPS units is around the 2008 era - new enough to have software-adjustable charging voltage but old enough to not have been "cost optimized" into low quality.
The normal problem with the older small APC models is that the charging / float voltage drifts out of spec to the high side, resulting in boiling / bloating the batteries. This can be adjusted on the older units and there is a lot of info out there on how to do it, for both the versions that can be adjusted in software and the older ones that need a hardware mod.
The larger old APC units are built like tanks, though - the system I'm typing this on is protected by a 22-year-old Matrix 5000 (with fresh batteries) and I recently started up an original Symmetra 16KVA that was purchased new in 2000, never used, and stored in a wet garage with small animals nesting in it and it powered up and ran just fine after replacing all of the batteries.
