The OP was asking about bidding, not selling. Clearly eBay sellers can set up 5-day auctions to avoid the problem. When eBay was young, I asked my LOR about bidding on eBay auctions that end on Shabbos, and he said that since the transaction isn’t finalized until you pay, it wasn’t a problem. This was before sniper sites and software existed, so I don’t know how to answer Shmoel’s question.
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