Received an offer to buy out my web app, what do?

Does he have a verifiable trajectory as an angel investor? Is he going to pay you in cash now?

1) Get your lawyer review the paperwork. Not his lawyer, your lawyer.

2) Is he going to pay you in cash now? Not equity, not a % of future earnings, not some money in 6 months. Green cash now. (Euros are fine too.)

Assume that this is all the money you will get. Are you happy with this amount? You may negotiate something else, but assume that you will not see the money.

3) Are there any strings? Do you have to keep working on this project? If you "quit", does he get all the money back?

4) Is there a non compete clause?

Let's be as negative as possible and suppose this is an scam and he's idea is to "invest" money paying himself as the CEO, some money to one of his friends as the CFO and more money to other of his friends as CTO, some peanuts for a freelance developer and some worthless equity for you that must do all the real work. Get a lawyer to be sure you are not getting screwed and get the money upfront for the same reason.

Way too many unknowns to give meaningful advice. However unless you believe that you've developed a money printing machine you can do the following: Say you spent 1 year developing it. Say you believe your fair salary for that year is XXX pesos and you've also incurred YYY pesos in expenses. So calculate the final price as (3.14 * XXX + YYY * 2 ) and offer to sell at this price.

Sell him some stake - build a clause that you retain ownership and input and control until he can prove he really wants to take it over, once you can assess the value he sees, then you can negotiate what to fully sell to him which works for both.

But don’t be scammed and also don’t look a gift horse in the mouth,

Think deeply about what you want, but do not sell 100% - retain 10% at least no matter what