Youtube knows I have subscribed to 515 channels, I have liked 2364 videos and favourited 685. It already floods the recommended videos with others based on “Users who follow<creator Y> enjoy videos of <creator X>”.
They for sure do not require my watch history to be able to recommend videos to me.
Because why not, maybe someone will buy it from you as they don’t know better, and you cash in the profit. It doesn’t cost anything to post something on eBay after all.
Some of them don’t even have the thing in the first place, if someone buys the listing they go get one from Amazon or a local shop and send it.
If you actually want to know what people are willing to pay for something though, look at the completed auctions.