Using Firefox, I noticed from Recommended For You, that Ebay knows what I do on Amazon and Amazon knows what I do on Ebay. I'm sure similar surveillance is widespread.
Solution? Use two browsers, one for E, one for A. (As well as using a VPN, taking usual browser tracking precautions etc.)
Extended solution? Compartmentalization. Use many browsers, each for a different task. (Great that Sparky has plenty to choose from in APTUS :)
However, when installing Chromium-based SRWare Iron and Ungoogled Chromium, I found that they both go to Google, despite choosing other search engine. Then after deleting both those, Chromium had Iron links in its menus!
So it appears different Chromium-based browsers share some common things and are not really separate. I imagine same would go for all Gecko-based (Waterfox, Ice Cat, Sea Monkey, Firefox) and Webkit-based (Otter, Falkon, Gnome Web, Dolphin) although I haven't tested these just yet.
My short-term solution is to use Brave (Chromium), Falkon (Webkit), Waterfox (Gecko), Pale Moon (Goanna), Mullvad and NetSurf for different tasks because I think they will stay compartmentalized and not share stuff with other browsers.
However, I don't know this for sure (I am not a programmer and do not understand the nuts and bolts of Linux)
SO MY SUGGESTION IS: keep an eye on what browsers share and aim for compartmentalization to make Sparky great for privacy.
Thanks for reading, sorry it became so long.