"Why can't all ring items show the measurements of the stones?"
It's not a requirement. I know it is helpful as a buyer to know this information but sellers are not required to provide the information in the listings.
If a seller doesn't answer your question about size, don't buy from that seller. If they want your business, they would respond like they should. You can't always get people to do what they should do.
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Non payers should get unpaid item strikes and be added to block bidders lists. Period.