I have a Cyberpower 1000VA which I bought used on Facebook marketplace for $40. The original owner said he thought that the batter was dead and it just needed a new battery so I bought it off him and ordered a new battery for it. After trying the new battery the UPS still does not switch over the battery when I disconnect the power cord from the wall.
If I get an APC unit instead (more of them pop up for sale on the used market) can I use the battery I ordered for a similar sized APC unit?
Did you give the CyberPower a good, long time to charge the new battery? UPS chargers tend to be small.
Regardless: there’s a decent chance that the new battery you purchased will fit an APC UPS. In this capacity range, a lot of UPS use a 12 Volt battery, of about 7 - 7.5 AH capacity, with physical dimensions of 6 inch x 3 11/16 inch x 2 9/16 inch, with push-on quick disconnect tabs. If that’s what you’ve got, look for a used UPS that takes that size replacement.
(That’s what I put into my CyberPower 900VA UPS - but it would have fit an old APC 600VA unit I have, just as well.)
(For that matter: an automobile jump starter I just repaired used a battery of the same size - but with different style terminals.)