For Reverse Auction when the bidding is in progress if any bidder places a bid of exorbitantly low price then it is not possible for others to bid.
In this case the auction may have to suspended and restarted.
Q). Is there any way that one can have data validation / business rule at bidders page for Bid Unit Price? Yes, In SAP Sourcing we do have a ceiling price but it only exists for the Dutch Auction.