you don’t actually buy it over the internet. you would contact the person over the internet and go meet them- no different then buying, for example, a car from a classified ad. you see the ad saying it’s for sale, you contact the seller, then meet the person face to face, look at what they’re selling, and then choose whether or not to buy it.
it gives you and the seller a chance to back out at any time, for any or no reason.