My 2c and experience on bearings is the different qualities make a difference. I've used ABEC7 grade bearings with ceramic balls and with stainless balls (separately), encased in rubber seals and with double casing. I've also used the ABEC 1 and 2 grades sold on ebay and local suppliers.
I found the ceramics to be much better quality and the rubber seal very effective at keeping dust and moisture out. They come pre-greased with Lube Kluber NB12/300KP Grease which is a good long term grease. Double casing will do a similar thing but make sure it's stainless or it will rust fast.
...but they aren't cheap and in the main need to come from BOCA bearings in the US, although local specialist suppliers might have them (here's a link:
https://www.bocabearings.com/product...os-7-nb2-16645). You can also get high temp rated of the same for about $2 more.
The problem with the ebay and cheaper bearings is that while they might do the job in the short term, they quickly develop wobble and wear faster than other bearings.