In Auction Style Drafts, FanDraft automatically prevents teams from overbidding based on your league's "Roster Size" rules, how many players are currently on a team at any given time, and how many bid dollars a team has left. This calculation is defined and displayed as "Max Bid".
If your league runs an auction draft and doesn't require rosters to be completely filled, and theoretically allows an owner to use all of their available salary-cap space on one player, you can accommodate for this situation by setting your "Minimum Bid" to $0.
This can be performed in the SETTINGS area, by changing the "Minimum Bid" to $0, and then clicking the SAVE LEAGUE CHANGES button:
In this scenario, you will also want to pay attention to your settings for roster size. FanDraft will automatically end your draft once all rosters have been filled, so make sure to set this amount appropriately.
If an owner has used up all of their bid dollars in this scenario, but your league doesn't allow these owners to continue to Nominate players during the auction process, you will have to manually skip this owner/change the nominator when their turn comes up.
You can do this by clicking the (...) button in the draft area, then select the "Change Nominator" option from the dropdown.