Events, spaced about evenly over 2 months:
Repeatedly ordered small amounts of webbing on a rather reputable Polish site (Allegro.pl, much like ebay).
Got targeted ads in Skype for webbing on Alibaba.
Clicked an ad (90% certain it was from Skype, else from targeted ads on some programming site) to see what the hell the image was showing; browsed for a few minutes. I was signed in on AliExpress at the time.
Received an unsolicited "we have webbing sling you may have interests" message, catalogue and quotation (80% complete order, just click "confirm order") from a seller on Alibaba
I have Windows 8 with ESET Endpoint Antivirus, Chrome browser, and gmail for the correspondence. All relatively up-to-date and showing no signs of being compromised. Purchases had been made on a computer with Ubuntu; no hijacks found.
That's all that feels relevant so far.
How did the seller know who to spam with this? Does Alibaba grant sellers access to a list of visitors? Was there some script on the listing or ad?