eBay intends not to let anyone else figure out what you’re in a habit of buying on the site. Because of that, lots of people consider eBay the obvious place to buy things you’d rather your neighbors not know you bought (there is a survey in this very paper confirming this fact). However, this paper demonstrates that a determined adversary can figure out what you bought.

(Caveat: This paper is from 2014. I do not know whether eBay has made any changes since it was published.)

eBay encourages both buyers and sellers to leave feedback on each other, the idea being to encourage fair dealing by attaching a persistent reputation to everyone. Feedback is associated with specific transactions, and anyone (whether logged into eBay or not) can see each user’s complete feedback history. Items sold are visible, items bought are not, and buyers’ identities are obscured. The catch is, you can match up buyer feedback with seller feedback by the timestamps, using obscured buyer identities as a disambiguator, and thus learn what was bought. It involves crawling a lot of user pages, but it’s possible to do this in a couple days without coming to eBay’s notice.

They demonstrate that this is a serious problem by identifying users who purchased gun holsters (eBay does not permit the sale of actual firearms), pregnancy tests, and HIV tests. As an additional fillip they show that people commonly use the same handle on eBay as Facebook and therefore purchase histories can be correlated with all the personal information one can typically extract from a Facebook profile.

Solutions are pretty straightforward and obvious—obscured buyer identities shouldn’t be correlated with their real handles; feedback timestamps should be quantized to weeks or even months; feedback on buyers might not be necessary anymore; eBay shouldn’t discourage use of various enhanced-privacy modes, or should maybe even promote them to the default. (Again, I don’t know whether any of these solutions has been implemented.) The value of the paper is perhaps more in reminding website developers in general that cross-user correlations are always a privacy risk.