Trading is copyright infringement, too. It doesn't matter what the trade is for, dollars or bitcoins or gold bullions or collectable saintpaulia leaves or other games. Trading means a significant share of public resources goes to agents who do not produce anything of value. Even on GOG, where the localized effect could have been mostly positive (people doing manual trades, spending time on the site and buying more games as a result), trading ended up being a significant factor contributing to the sorry state of the forum. "Professional" traders are causing measurable harm. And if, for example, Konstantin Vorontsov [please hire me, sir!] decides to skim the cream off the whole industry with automated trading, it'll go belly up.