Can't charge Apple Watch 6 - is there any personal data on it after unpairing it?

My Apple Watch 6 stopped charging. I brought it in for repair assuming it just needed a new battery. It had to be sent offsite, so the employee had me unpair the watch from my phone (not sure how it unpaired with no power, but it did). After it was received at the service center I was told that the repair would be $299 (the problem was the charging coil, or whatever the charging element on the back of the watch is called). I declined the repair so it was sent back to the store and I picked it up. I'd like to try to sell it to someone can use it for parts or repair it themself, but I don't want to risk having the buyer able to access my personal information or other data. Would unpairing it have erased everything (even though it had no power) to a point where it cannot be recovered, or am I at risk of someone being able to recover the data, in which case I won't bother trying to sell it? Thanks!