In principle, I don't think you identify a shill bidder from the
history of a single auction. There is not enough information there to
distinguish a shill from any other bidder.
I think I can explain the bids you found puzzling, as noted in your
clarification. The key is to look at the times as well as the
For example, let's look at bidder6 raising the price from $10 to $20.
If you just look at the prices, it looks like he is just bidding
against himself for no apparent reason. (I will use male pronouns for
simplicity.) But if you look at the time sequence, you can see that
first, bidder14 bid $20 at 11:45:30. The previous high bid was $7, so
bidder14 would have shown as the high bidder at $7.50. At 12:57:05,
bidder6 comes along and sees a minimum bid of $8. He boldly bids $10,
but because bidder14 had bid $20, bidder6 is automatically outbid, and
the price goes to $10.50, with bidder14 still the high bidder. The
minimum bid is now $11 and bidder6 decides the item is worth that to
him, so he bids $11. Again, he is automatically outbid and the price
goes to $11.50 with bidder14 still high. So bidder6 tries again with
$15 and is again automatically outbid. So he tries $20, tying
bidder14's bid. Now the price shows as $20 with bidder14 the high
bidder, because he bid that amount first. Bidder6 can now conclude
that he has tied the high bid, so he bids $24 and becomes the high
So bidder6 was not pointlessly bidding against himself all this time,
he was bidding against bidder14 and finally outbid him. This is
actually a common pattern on eBay. It would be simpler just to decide
what an item is worth to you, bid that amount, and let the automatic
bidding work, but many people like to bid small increments repeatedly.
It's not uncommon to see several bids from the same person a few
seconds apart, all losing.
The case of the winner bidding the price from $90 to $100 appears
similar, although the transcript looks a little fishy in showing two
$100 bids from the same person at different times. But anyway, while
the winner was bidding the price up, he kept getting outbid by the
earlier $115 bid from bidder4.
The case of bidder1's bids from $75 to $80 does not fit this pattern,
but it is not necessarily suspicious. bidder1 could have been
changing his mind about what the item was worth to him and increasing
his bid for protection from other bidders.
To identify shill bidders, I think you have to look at patterns across
multiple auctions. If bidder A is always coming in second in seller
B's auctions, that would be suspicious. (The idea of a shill is to
bid up the price and lose to a real bidder.) If bidder A has no
history of ever buying anything else, that would be more suspicious.
If bidder A bids on all of seller B's auctions and never bids on
anything else, that would be suspicious too.
For more on detecting shill bidding, see the last answer on eBay's
help page on the topic:
eBay says it is suspicious for a bidder to make repeated bids in small
increments when he is the high bidder. As I said, you have to check
the times to determine whether a bidder is actually the high bidder
when he makes such bids, or he is bidding against a higher previous
I hope this explanation is helpful.