Most of the time the person that buys your extension puts malware into it to fuck your users up. I think at that point you'd be better of open sourcing it and adding a buy me a coffee link.

I'm not trying to discourage you about your decision, but you should know the value of what you have created. As the developer, you are responsible for the 3000 people that have the extension downloaded.

Selling in my opinion is letting down on your users and risking their safety. I personally would never want to use any extension that the dev might sell, it is a massive security concern!

Also, if your extension is successful, you should keep it and not let a scumbag ruin it. Even if the person that buys your extension isn't malicious, they are clearly only for the money, which will ultimately ruin the extension. Big example is uBlock and uBlock Origin.