In Admin => Edit Categories when setting up the category you set whether to allow bidding or not.
When someone places an ad they can check to allow bidding or not. This will tell whether it is an ad or auction. If it is an auction then users will see the bid controls and be able to place a bid. If they can not see bid controls then the ad was not marked to allow bids.
Feedback can be left in a users profile usually the link to leave feedback is sent via email to the winner of the auctioned item.