Bids concentrated with this seller

What it looks like on eBay

• On the bid history page, tap a masked bidder name such as a***b (12). eBay opens that bidder’s detail page.
• The detail page shows a 30-day summary: total bids, items bid on, and the percentage of bid activity with this seller.
• A bidder with 40 bids in 30 days, 96% of them with one seller, is the pattern this sign looks for.

Why it counts

Ordinary bidders spread their bids across many sellers. In the UCI Shill Bidding Dataset (807 eBay auctions), almost all bidders scored near zero on seller concentration, which makes a high share one of the more distinctive patterns in public data.

Innocent explanations

A present sign is a reason to look closer, not a conclusion. Common innocent reasons for this one:

• Niche collectors follow one seller who reliably lists what they collect, such as one set, one player or one grading company.
• A new eBay account may have bid with only one or two sellers so far, so any share is high.
• Some buyers watch a single store’s weekly auctions and bid on most of them.

Check it yourself

1. Open the listing and tap the bid count to open the bid history.
2. Tap each of the most active masked bidders.
3. Read “Bid activity with this seller” in the 30-day summary. Note the total number of bids next to it: a high share of 3 bids means much less than a high share of 60.

Read more about shill bidding and how to spot it, or see how Susbid decides a sign is present.