FYI - grey market does not mean refurbished or repaired. They are brand new, standard sets.
The way the grey market works is:
1) Manufacturer and/or distributor offers quantity discounts to retailers
2) Authorized retailer buys more than they can sell at retail in order to get the quantity discount
3) Authorized retailer sells (unloads) the excess units to a normal, aka "unauthorized" retailer at a discounted wholesale price
4) Normal retailer is free to sell the units at whatever price they want
Sometimes you'll get grey market stuff that was intended for a different country, but that's pretty rare and usually disclosed up front because while the retailer is not "authorized" they aren't criminals, frauds or anything like that. They just didn't sign a contract with the manufacturer to keep prices high. It doesn't hurt to verify that it is a US model though.
Also, FWIW, the manufacturer generally knows what's going on. Their sales reps know their customers and when a customer places an inordinately large order, it is obvious that something is up. An honest manufacturer will refuse to fill such an order. A dishonest manufacturer (most of them) will happily fill it knowing full well that some of the units will flow to the grey market making the customer responsible for the hassle of figuring out the unit's provenance.
My opinion is that if there is a significant grey market for a manufacturer's product then any denial of warranty service is hypocritical and immoral since it is almost certainly their fault that there are any grey market units to begin with. But what's immoral and what's illegal are two different things.