I would suggest you post a url and a test login. This way we can interact and see your issue.
I do not show any issues with bidding in the current setup. Here is how it works.
1. You in Admin Edit Categories set categories to allow bidding.
2. User uploads ad and ticks allow bidding. This must be done for it to be an auction.
3. If it is an auction then logged in users can bid on the item. This code has not changed in quite a while
To show you the process I have a link here to a new test setup where I uploaded a test ad and did the steps above. I went to the ad and in the bid table entered by bid. Feel free to register a test account and place a bid. http://www.omegatron.net/classifieds....php?product=9