There are several dozen Minecraft accounts that are being sold that are non-premium and cannot be upgraded. They are 100% almost all worthless and some people are trying to sell them full price.
Some retard members got their hands on the ones that weren't premium and keep trying to sell them.
If you want to know if the account is premium or not, just get them to join a server on it that's not cracked, otherwise anything else can be faked and screenshotted in the MC client.