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?
Depends if it uses weird connectors. Some UPS use normal F terminal connectors and some UPS battery packs like APC occasionally use plastic slide in trays that hold batteries and have goofy proprietary connectors. The batteries inside the sled can be replaced with the normal SLA batteries that are commonly used in UPS units though, just reuse the sled.