To sell the forum you could probably sell it quickly on a site that has high volume and has a very active websites for sale forum. I'm pretty sure that if you put it up for sale on
sitepoint it'll sell quickly. There's a 10$ fee but you could include it in your price. Make it into an auction and start your price at about 300$ and state that the bid increments should be at 10$. You should also set your bin (Buy it now) price at around 500$ so if any one wants to buy it right away they can. That is of course if you're planning to sell it at that price.
There are a few things to consider when evaluating your forum.
- Number of members
- number of backlinks
- Monthly revenue
- PR
- age of the site
- Traffic
- Activity
If you make your the sale of your site into an auction you could possibly sell for more than 500$. I think that 300-500$ is definitely do-able. Remember, potential doesn't sell, it's what you have right now that will sell.