Bid Now transaction is an auction transaction where the user places his bid on the product of his choice - once a bid has been placed, the user has a chance of winning the product if certain conditions are met e.g.
If the auction is closed
If the reserve price is met (in the case of a single inventory auction)
If his bid either the highest bid (in the case of a single inventory auction) or greater than the lowest winning bid (in the case of a multiple inventory auction)
If he is a winner once all the above conditions have been satisfied, then he is intimated by email about his win from where on he can place the order for his product.
In the case of a Buy Now transaction, he can immediately purchase the product at the fixed price at which it has been made available.
Obviously, shipping time applies for both of the above:
Bid Now: Delivery time = Auction Duration+Order Processing+Normal shipment time
Buy Now: Delivery time = Order Processing+Normal shipment time