I have just bought and downloaded the Mac versions of  Fugue in Void and Arkanoids. Neither of them will load; I just get an error message saying there is a problem. I can sort of understand one developer screwing up, but two in a row? I get it that Mac versions of games can be a pain to produce, which is why so few games bother to do so, but if they can't do it right, they shouldn't even try. And they certainly shouldn't be marketing dud games... I'm now very dubious about buying any other games from this platform.

I am using Mac OS 10.13.4 High Sierra.

I believe that it's because they don't own actually own a Mac to test it. And,  because they have the option to make a Mac version with the engine that they are using, they thought that if they did it then it will most likely work because their windows export of the game worked. Because this site is filled with games made by one man teams, some people just don't have both platforms to test things. Hopefully you contacted the dev and tried to get it to work. 

If anything, you can try and refund it.

Your frustration is justified, but Itch.io is only the marketplace. Most of the responsibility lies with individual creators. When you have such a problem, you should talk to them first; Fugue in Void for instance has its own community where you could complain. (Couldn't find Arkanoids, there are many games with similar names.) If your problem remains unsolved, it's always possible to request a refund via the support e-mail. But as WallaceLovecraft points out, it's usually a matter of not enough resources, as opposed to lack of interest, or worse, malice. And I know for a fact that some creators here actually own Macs and even develop on them, so their games at the very least should work.

I think it should have been obvious that I was puzzled rather than angry; I certainly don't want to claw back the 2-3 euros I paid for Arkanoids. But I do feel that if the creators haven't tested their product on a Mac, but are just guessing, then they shouldn't advertise such a version at all; this is perhaps something Itch.co might consider imposing as a general rule, since repeated failures will inevitably reflect badly on the site!

maybe itch.io could add "Not tested on Mac/Windows/Linux OS" next to the download, this can help developers if the game could not be tested for that OS. Than after it been tested by players who played it the OS tag will be ok for everyone for that OS.

But we don't test all the games. We lack the resources, and it really isn't part of our mission. Steam could... yet they allow games with no executable in the package at all. We'd at least contact the creator if something like that was reported here.

And are you sure those are repeated failures? If other people can run the same games you can't, then maybe it's your machine. Happens all the time, on any operating system. There's too much diversity out there to guarantee that every game works everywhere. Even in the 8-bit era, when in theory each home computer had one fixed hardware configuration, that wasn't quite true, and such things simply happened.