Mini rex should have very short velvety hair, where the guard hairs are about the same length as the undercoat so that when you pet the rabbit you can only feel the soft undercoat. In pictures, you can rarely see the fur texture of the rex fur because it is so short and fine, and they also usually curled whiskers which your rabbits do not have. They also do not have the right fur for a Silver fox. Silver fox have standing fur - which is very long and thick compared to normal fur and resembles the fur of an actual fox. Her color looks like a silver marten to me; in the UK that's called silver fox (and has a breed by the same name) so maybe that's where the seller got confused.
His color looks like maybe a broken blue otter? Hard to tell from this picture. The older babies look like a blue, booted/vm black, broken blue otter and blue otter. Can't tell on the younger ones yet. They're definitely mixed breeds
