Ebay should allow buyers to bid for a single item across multiple auctions: make a bid for one item, then if outbid, automatically make the same bid on the next identical (as defined by the buyer) item and so on. This increases efficiency by joining multiple auctions for identical items into one market with many sellers and buyers. It also reduces selling times, because a buyer who just wants one unit does not have to wait until being outbid before bidding for the next identical item. Buyers generally are not continuously watching the auction, so there is a delay between being outbid and manually making the next bid. Buyers are willing to pay to reduce the delay, as evidenced by purchases at “buy it now” prices greater than the highest bids in the auctions.
More generally, bids conditional on being outbid would help merge auctions into markets, gaining efficiency and speed. For example, a buyer has different values for used copies of the same item in different condition and wants just one copy of the item. Conditional bids allow the buyer to enter a sequence of different-sized bids, one for each copy, with each bid in the sequence conditional on the preceding bids losing.
Linking the bids is not computationally difficult because Ebay already sends an automatic email to a buyer who has been outbid. Instead of an email, the event of being outbid can be used to trigger entering a bid on the next copy of the item.
Faster selling times benefit everyone: sellers sell faster, buyers do not have to waste time checking whether they have been outbid and then making the next bid, Ebay can charge higher fees to appropriate part of the increased surplus from greater efficiency. Ebay can also use the data on which items buyers consider similar enough to classify products and remove duplicate ads.
A browser extension or app can provide the same functionality: an email with title containing “You have been outbid” triggers code that logs in the user (with the credentials saved into a password manager or the browser) and types in a bid on the next copy of the item.