Good: You can create a forum creation service and be paid for making forums for people. This saves people alot of time and therefore they will request something like this.
Silly: Why not keep the forum for yourself? If you don't want it, give it to a mate or something.
Elf - Because it's people's jobs to do this sort of thing, it's called Site/Domain Flipping, basically people buy and create sites, let them age, sell them off for a profit, or they buy sites, do them up a bit, and sell them off for a profit.
You should look around forums like Digital Point, you would be surprised with how much this happens.
I've made allot of forums, most just testing forums, or forums that never lifted off. I've never considered selling them, but now thinking about it, I'd be willing to make a forum and sell it. Forum making is a hobby I do when bored.
I don't see anything wrong with it. I've bought forums and I'm happy to see the people who did the initial forum building work get paid for their efforts.
If people can earn some money online by building and selling forums, then why not do it, there's nothing wrong with it?
From looking at DP there are always people looking to buy good quality forums.