he was already bid in your player (when he had a + finance).
He can't bid in a new player now that he is - .
Many managers choose 5-10-15 players from the list and bid one T. The value of all those players is bigger than their budget but as they already bid on them, they can finish those auctions.
After that, they can choose to sell some players (or maybe some 4* players from previous season are already for selling)
or to take money from the bank.
As about last sec bid, there are many managers who bid that way.
* most of the time it's stupid
All the experience managers and forum members can confirm that there aren't bot bidders but there are so many crazy bidders (so nordeus doesn't need that option).