IN-GAME TRADING
TradeItems() implements limited in-game trading of items, if you prefer not to use
the overlay or an in-game web browser to perform Steam Trading through the website.
You should implement a UI where both players can see and agree to a trade, and then
each client should call TradeItems simultaneously (+/- 5 seconds) with matching
(but reversed) parameters. The result is the same as if both players performed a
Steam Trading transaction through the web. Each player will get an inventory result
confirming the removal or quantity changes of the items given away, and the new
item instance id numbers and quantities of the received items.
(Note: new item instance IDs are generated whenever an item changes ownership.)