A very good question. DGMarket is based on the Steam platform architecture because it is (probably) the only platform that provides the "API", a system that developers can communicate with.
In order for DGameMarket to support other platforms, you have to add a lot of games manually. It's possible we'll do it someday.
Remember that the games included in this post are a different project than games in the market.
Yeah just noticed that many of these are UPlay games, and they can't be entered on that platform... Good idea either-way. Keep it up!!
Ah, I recommend putting @steemshop tag #steemshop, they aim to offer higher visibility for those who are selling things for steem!