Reputation works this way.
Lets say you make a post
Someone thinks it is a great post they will click the reputation icon to add to your reputation
If they don't like it they can choose the disapprove reputation and it will come off your reputation.
It would include another image underneath everyones avatar.
This would show how trustworthy or intelligent someone is with their posts, or how untrustworthy or stupid someone is (don't yell at me for the choice of words i make here it is generic)
Administrators can change the level of reputation and everything. Basically if you just had it for the marketplace and not for people's posts you could tell how worthy they are to make a deal with for a computer or some other random device etc. etc.
Attached are images of which i mean
First Image: Admin Picture of reputation stuff
Second Image: Picture of reputation clickable icon
Third Image: What you would go to after you click previous icon
Fourth Image: Picture of what generic Reputation looks like (default vB) Someone with more reputation will have more green icons there
FYI: images are from my site where i just turned on reputation to give this walkthrough. No i do not use reputation on my own site
EDIT: Second Image is under the other three since it is not needed as a thumbnail (just found a feature of vB i don't like

)