Output Size and My Gumroad Refund Policy

Someone bought my substance from my Gumroad, and shortly after I got this message:
“This is not how it looks like in photo.  This is disrespect to other people work. I ll never buy something from you”
This happens in approximately every in every 100 purchase. Mostly I get complaints that the substance seems “low resolution” or “blurred” and sometimes vague comment like this.
Now, I can guess why this happens; this got to do with Output Size. The default parent size is 256*256px in Substance Designer. I save and publish my substances relative to the parent as this is the correct way to do it. Check out this page in Allegorithmic’s site.
Now the above statement is vague at best. In order to get the substance look like in the preview render one would need to adjust render settings. The person does not explain what is wrong with it, just that it is disrespectful somehow.
I do give refunds sometimes but user not knowing how to use Substance Designer is not basis for a refund.