Sure you can sell it for $30 or $50 or $1000, but why would anyone buy it?
There is no return for the buyer in the situation you have provided.
PR5 means little. PhpBB won't add any value to the forum. Posting for one month...with no other members? Remember, you need to also get people to post on your empty forum.
Buyers want a return, they want people to visit their sites or buy their products. If you can't even achieve that, how do you think a buyer will respond?
How many forum posts do you get daily? How many posts are on forum? What's your daily traffic/page views? A webmaster will want to know all this before considering advertising.
PR is pretty irrelevant nowadays. It has potential if you get a good, already established domain but after a month? I highly doubt it. Your work will be nearly the same as a new domain apart from that search engines may index you sooner.
