Hi,
I think that’s this is a question only you can answer for yourself. 3500€ is a good amount of money, but the site you are selling can achieve this sum in about an year, perhaps a bit more. As your earnings are progressive, I am assuming that over time, the monthly income will raise for sure.
If you consider the site, something you are no longer interested in, or perhaps want to head in a different direction, then sure sell it, get the money and invest them in whatever you want.
However if you have put a lot of effort in it and you enjoy working on it and it makes you happy that you see results and revenue, I don’t think you can put a price on that, and I would suggest to pass on the offer.
Bottom line is that you should listen to yourself, as someone else on the forum, might give you a quick summary and their own opinion like myself, but we have no idea the trouble and effort you have went through and how valuable that site is to you.
Hope this helps.
Regards