Here all bidders write their offer price to buy the item for auction and these bids are deposited in a box. Before moving to other item for auction these chits are opened and sale goes to the highest written bid. These written bids are final bids and sale occurs as per them. So, basically spot bid is a written bid.
Spot bid auction is generally done for selling of government properties. However this method of auction is hardly used for auctions.
There are many agencies which hold an auction after certain intervals, when the products can be collected & later displayed for an auction. Auction items include office stationery, laptop, computer / accessories, clothes etc.