if you make a texture in 256x256 and upload it, does the texture have better quality ingame then a 512x512 texture?

Yeah it does have a better quality

Well as far as I know the textures are re-sized when uploaded to 128 by 128 pixels.
So if your picture is 512 by 512, it shows more quality, so the resized picture will show more quality I think.

I might be bambling though, I'm not quite sure about this.

No, the higher quality the picture is when you start, the lower quality it will be ingame according to this thread: http://forum.toribash.com/showthread.php?t=84228

It doesn't matter what you upload, the game will only use it at 128x128.

Novi's thread was to make people aware that when you see a texture for sale you have to be aware of the size. If it's 128x128 then that's what it will look like in game. If it's 256x256 then it'll be 50% blockier/blurrier. 512x512 will looks 75% worse...

There are exceptions to the rule, and it mostly depends on the texture artist.

It is best if you do not use the texture size 512x512 or 256x256. And always use the size 128x128. So you do not lose quality in texture and not to mislead the buyer of your textures.