Imagine you had a second-hand laptop you were looking to sell. You could see what the going price was on a site like eBay, list it and pay a fee, then wait for a buyer to bite. But if someone you knew offered to buy it from you directly, then you could save yourself the hassle and the fee, and potentially end up with a better all-in deal.

If everyone did this, though, the next time you came to list a laptop, you would find yourself with fewer data points to work out what a fair price was. And