I have done that a few times in the past. There were times where I regretted it and times where I thought I had a very good deal, but I think the important thing to get out of it, is the experience. This experience will really help me if I decide to sell my website in the future. The reasons why I sold my websites in the past was due to not having enough time to manage it and also getting good offers. At the end of the day, I wouldn't sell off a project if the offers were bad even if I don't have enough time to manage them.
I've created three forums, in which all was very successful start-ups. However, due to my in real life job, I struggled to find the time to continue and manage it, since I was working on my own.
I spent over $3000 on advertising, paying people to post, custom themes, graphics, video advertisements, custom plugins and more on one of my forums. In which I later sold to someone who was active and who I felt would take it far and continue my legacy. I wasn't looking to make profit, i only wanted the forum to continue. I sold it for a total of $800 (even though i had much higher offers, I just didn't feel comfortable in selling it to someone new) I'm proud to say the forum is still online and is holding in strong.
I won't be selling my recent forum I launched two days ago, though! This one is here to stay in my hands.
Can't say that I've ever sold a forum before or that I'd be interested in doing so. I guess if I didn't have time and someone offered a worthy amount, I may not be able to pass up an offer. The likelihood of that isn't very high though!