Used Apple watch wont pair

Its a story... it was my watch, Series 3 with e-Sim. I did not use for about two years, decided to sell. Unpaired it, removed from my account, deleted all traces, complete reset, came back on as new watch.

Now, buyer tries to pair, it pops up pairing when close to his iphone 10 (latest IOS), goes to pairing (camera image of "cloud" on watch face), then says "activating watch". After some time, says unable to pair.

Try again, same process but manual, entering code from watch to phone, seems to process then returns to "Set Up Apple Watch". Tried hard reset of watch, same, tried different phone (iPhone 7, latest IOS), same, read all advice online, same....

Totally stuck..