Customer Offers can be attached with Auctions, and are valid only after auction is closed, to target those buyers who were outbided during the auction.
If you are a bidder who has won an auction, then there is no custom offer for you. however if you are outbid by someone then once auction is closed, you will receive an email and Facebook notification to buy a follow up Buy Now purchase. This will be a limited time offer and you must buy within that time frame.
There is no difference in your experience of bidding in an auction with or without custom offer. In fact you will never come to know until auction is closed that there is any such offer seller has setup.
The experience of Buy Now custom offer will be same as you would experience in Buy Now, refer section "Buy Now"
Features -> Custom offers
1) Owner can create Custom Offer and it can saved in draft/edited/ published.
2) Custom Offer is similar to buy now listing and includes Time Valid for : Immediately when closed, 1 Hour, 2 Hours, 3 Hours, 12 Hours, 24 Hours, 48 Hours, 72 Hours, 5 Days, 7 Days and 14 Days and notified hourly basis.
3) Once the custom offers are created, all the Custom Offers are displayed in the custom offer grid under columns names : Listing title, Time valid for, SKU, price, Status and Quantity available.
4) Custom offer can be sorted based on status : Draft, Current, Sold, Unsold) in the grid is based on the under bidders brought the offers.
5) Emails ( "Your custom offer has arrived") and Facebook notification is received to all the under bidders, if bid range is not checked by owner. If Custom Offer is set to Time valid for : 1 Hour, then the offers is notified immediately and will be valid for 1 hour of listing sold and so on.
6) Underbidder can buy the custom offer within the valid timeframe. If underbidder buys the custom offer then the status is changed to sold otherwise after the closing time, it is closed.