There is a systematic way to establish with a high degree of certainty that the user is indeed a human being. After that, you can only go by what that user does.
Depending on the nature of your web app, "bad" users will probably be those who:
- Try to dump links for SEO and run
- Create sock puppets to promote (or demote) another user
API's like Akismet let you catch a significant amount of SPAM before it even shows. Beyond that, you should have a means for "good" users to flag the post / auction / whatever as being inappropriate.
Sock puppets are really nothing new, and are discovered by analyzing usage patterns. Rooting them out on a site like Wikipedia is a bit different than rooting them out on a site like SO, where someone's 'normal' use is quite deterministic. That means misuse, (abusing the up / down vote buttons) is easier to catch. Its hard to give specific recommendations without more information on what 'good' behavior would be on your site.
What I would do is offer some kind of reward for the behavior that you want. If you want people to post items for sale, give them a star or something next to their name after they've done that for a few times. Reward the 'star' with something else, like reduced fees, reduced advertising, whatever. Make it worth while for "good" users to give you something in order to get something.
If your site really is that 'disposable' in the eyes of your visitors, you'll get a lot of drive by users. Perhaps, think of a way to make people want to participate over the span of at least a few days, this will give you (in most cases) the time that you need to root out the bad elements and reward the behavior that you want.
Stack Overflow itself is 'disposable' to some of its users, there are many who never actually register, ask a single question, get their answer and vanish as soon as their cookie does. But, the quality of what they do while there determines their credibility, and how likely they are to get what they want.