Yep, I third (?) that : chinchilla without a doubt, but what kind of 'breeder' doesn't know how to recognize a basic colour? (that is also pretty telling regarding the fact that the breeding wasn't well-thought out - a breeder should pair rabbits because they want to obtain certain characteristics and thus know beforehand the probable outcome of the breeding). I'm not very fond of the wire mesh on the bottom of the cage either - it hurts the rabbit's paws in the long term, it's what the industry of meat rabbits use.
So, the rabbit's cute and all but just... make sure to take a very good look at all the rabbits there to check they are healthy, no parasits, no scabies, no sad/hunched looking bunnies before taking the rabbit home. It can't hurt to be cautious when you're dealing with a backyard breeder.
I'm not knowledgeable enough regarding mini rex to talk about the quality of the rabbit in terms of showing... If you're not planning on showing or breeding, it doesn't matter much anyway.