Digital goods exchange script. Does anyone have an example script? Even if at a primitive level?

Question

Data exchange

Good afternoon/evening everyone!
Guys, we need a simple script for exchanging digital things like a game for a game, a digital book for a digital book.
Who has, or who can just show an example of a script? Please help me create an example script.
Site example: there was one site where dota players exchanged their things among themselves. Tradeit and something like that.