I use a decision tree to determine which platform to buy from, by going down this list and selecting the first platform that fits my requirements.

1. Steam: If the full game is on there, buy. If the game needs an extensive patch to provide all the content, buy from another platform.
2. itch: If the full game is on there, buy.
3. GOG: If the full game is on there, buy. If the game needs an extensive patch to provide all the content, buy from another platform.
4. JAST USA: If the full game is on there, buy.
5. DLSite: If the full game is on there, buy.
6. DMM: This platform is annoying to buy from since they dislike foreigners. If the full game is on there, buy.