It all depends on its existing income and what it is doing right now. I have built a script that gives a pretty accurate appraisal in my opinion. http://www.priceasite.com to get a free one just to see what you think. Normally, I would NEVER pay much more than what is listed there. Then again, some others might.
Depends are they active members? If so you might have something there and you should be taking advantage of an email list like that . hehe
That site is not even remotely close to evaluating the worth of online property. The fact you state that you use it as a main determining factor, instead of the true metrics, is very sad. Shoemoney has typed out some tips for evaluating some online investments at his blog. You might want to give that a peek. I can tell you that there is no true website anywhere on the web that will give people a true value of a domain, or site. So do not rely on them. 2 cents
It depends. Because it's a PTC site, I'd have to look at the monthly profits instead of the amount of members. Now, a forum is something else.