Is there no demo version available? No specs given? No OS? Most stuff on itch runs on a potatoe.
And since practically all stuff here is drm free, a general refund policy is non existant. This ain't Steam.
If there are serious issues you can contact support for a refund.
Many games give information about the engine used. If you know that games made with that engine run on your system, you have no cause to suspect that the game will not run on your system. For a thing like renpy you could even download the developer kit and make it be able to run on your system, even if the developer did not bother to create the version for your os.
And if you buy things that do not run despite being correctly labeled, you should contact the developer first. Many things are in development here, so there might be problems that could be easily fixed.