Can I ask how it went legally wise? Did you involve lawyers etc?

At least in the US, hiring lawyers for a transaction less than US$100K is very problematic and generally does not make sense.

There are some other legal options (like those express legal advice websites, or places where they proofread the agreement for US$200).

Yes, that's why I asked if they used lawyers for this relatively small sum. But it's not minuscule, so I was interested if they used contacts/agreements, escrow or something like "you send me money I send you password for account". Would like to read about such small-scale legalities for small personal sites-startups if it's available anywhere.

Try negotiating a fixed sum for the drafting the agreement. I think under $3K - $5K it can be doable, but also do a lot of sanity checks, e.g. provisions that you get everything back if the money is not paid in full, etc.

Professional lawyers do make mistakes, no matter how much they are paid.

Which is why you shouldn't feel sad. You probably couldn't have realised that value anyway, unless you had those skills.

There's lots of room between "don't feel bad" and "zomg, I missed out on $12.5 million", room for, "hmm, how can I do better next time, how can I add value to something I own."

Interesting. Are you allowed to tell us more about what the site did and what the buyer did after?