My gut feeling from what you've said suggests that they may just be reselling accounts on a domain that they own, rather than being a full G Suite reseller. I'm not completely certain, but I don't believe the "This account is managed by..." appears if you've got your own G Suite account, even if it's being provided by a reseller.
I'm also a bit concerned that they don't seem to appear in
Google's Cloud Partner directory, although they might be listed under another name (I searched for GoEz, Zambroni, and The Six Group and nothing came up). However, none of the partners listed in Miami are at the same address either.
I also find it weird that they want an eBay username on their signup form.
One way to double-check this is to try to login to admin.google.com using your account. While a G Suite reseller will often have access to your admin console as well, you should have primary access, and in fact depending on the reseller relationship, you may even be able to disable their access. See
Google Reseller access to Resold domains. If you can't login here at all, then you're basically just an end user on their account.
Personally, I'd say that unless they can provide confirmation that they're authorized by Google to be doing whatever it is that they're doing (which in most cases, would require them to be an official Google Cloud Partner), I'd steer well clear. For the buck a month in price difference, you're far better off to just sign up for G Suite directly, as Bill suggests, especially since if they're just reselling mailboxes from their own account, they're in violation of the
G Suite Terms of Service, which means that you're taking the risk that Google could shut them down at any point.