I do not replace Apple Watch screens. The latest and most expensive Apple watches cost ~$750 new so it seems like there would be good money in them. However, the only good replacement screen kits available are from Apple and they cost over $300. The replacement is complicated, and should cost ~$150. Realistically, for the kit plus the repair would be $350 + $150 = $500 and in the end the customer has a watch that is probably already “dated” by Apple product perception, and it may have been damaged by whatever cracked the screen, and it has been opened by human hands when it was not designed to be opened at all. To replace a screen, the glue behind the screen has to be cut and replaced with new glue. It is just not as good as new from the factory, built and touched only by robots.
Maybe it is worth it for the older models when the aftermarket screen kits catch up after a few years? Maybe, but then the final product is “dated” and “only” worth $300 in the end and even generic screen kits cost over $100.
On top of all of it, buying used Apple watches is complicated by the security measure. If the watch is linked to an iCloud account, it cannot be released without the rightful owner (despite the various claims on the internet from shady people willing to take your money to let you try their software and fail).
Bottom line: the second-hand market competes with Apple’s main income stream so Apple watches are designed to go out of date and drop in price before the second-hand market can catch up and compete. In summary, do not crack the screen on your $750 watch unless you can afford a new one because the repair costs almost as much.