If you go to buy a forum first time then please ask these questions.

1. How much new visitor you get per day
2. How much live user on that forum
3. Whats platform you used to create this forum
4. If you doing paid advertising on forum then how much revenue you earn per day.
5. average post, comment posted on forum
6. Alexa Rank (Its help you to aware about the popularity of forum right now)

These are the question which help you get a best deal if you go to buy a forum first time.