To try to answer the original question, I would guess that what most customers are looking for in an "overwintered" nuc is a queen who's proven herself over a winter, along with bees and brood that are her progeny, as opposed to a mishmash of resources put together into a nuc right before sale. I wouldn't think that having overwintered in a small box would matter.
Whether this is something customers should want is an interesting question. My local beestore sells both "regular" and more expensive "local overwintered" nucs. I've purchased both and will describe my personal experience, for whatever it's worth.
The regular nucs came in a homemade 6-frame plywood box with a frame feeder full of some homemade mix of pollen sub and sugar. Three of my colonies were perfectly fine, while one had to be requeened immediately. All were healthy. Most eventually swarmed because of my incompetence and lack of drawn comb, but otherwise they were great bees. The one I immediately requeened made excess honey and all survived the winter.
The local overwintered nucs came in 5-frame coroplast boxes. Both of my colonies had run out of protein stores and had begun eating their own brood by pickup day. Once fed, they built up great, and had no health or queen issues. Great bees to work. Both stored excess honey, eventually swarmed, and so far appear to be making it through winter.
So, I don't know, I guess my limited experience support's dudeIt's suggestion to buy any healthy nuc you can and plan to requeen it. My purchased queens have been the best, followed by the queens that came with my nucs, while the queens my bees have made for themselves have been my least favorite, mostly because they've been more defensive.