What issues are there with buying points on the resale/grey market as opposed to buying points direct from Disney?
As mentioned already, there is nothing grey about the resale market. It's totally legitimate and there are several very reputable and experienced brokers that folks here have used to buy a resale contract.
Would we have two home resorts if we bought points direct from Disney for a different resort?
You would have two home resorts but you would not be able to combine your points to book one of your home resorts 11 months out. Just being an owner of a particular resort does not give you 11 month booking privileges. The points carry the booking privilege. If you want to book BWV in the 11-month booking window, you need enough BWV points to pay for the reservation. I don't own at the Poly but if you transferred your Poly points into my account, I could use them to book the Poly 11 months out. She who holds the points gets to book 11 months out, owner or not. If they allowed owners to use any points they had lying around to book in the 11-month window, it would create a situation with way too many points chasing too few rooms in the 11-month window.
Is Disney selling used points and where?
There really is no such thing as "used points" or a "used contract". Some people think you get 50 years when you buy directly from Disney and just the remaining years when you purchase a resale contract but that is not true. The end date of each resort is fixed, no matter whether you buy from Disney or on the resale market.
And Disney does resell contracts that they have acquired through foreclosures and by exercising Right of First Refusal, when they step in as the buyer when an owner sells a contract on the resale market and they want/need that particular contract back for some reason. But those resales are full retail price of course. Your Poly contract could be a resale of a contract Disney got back due to a foreclosure.
If you buy from Disney vs resale, you stay in exactly the same pool of rooms as everybody else. They don't put direct purchasers in newer rooms and resale purchasers in older rooms, it's one pool of rooms. If you purchase a new resort from Disney and the Residential Unit to which your contract is tied just opened for occupancy, then you own a piece of a brand new room but since you would rarely if ever get assigned to the Unit you own, I (a non-owner with some resale points) might be staying in "your" room and you might be staying in the oldest room in the resort. It's just the luck of the draw and direct vs resale is not a factor in room assignments.
The only differences between purchasing directly from Disney vs purchasing on the resale market are the perks that you get, the time the transaction takes and the price, which can be as much as 50% lower when you purchase on the resale market. There is one other difference. Disney's contracts come with a full set of current UY points and all points going forward. Resale contracts could have the same, more or fewer points depending on how many points the current owner used before putting the contract up for sale. They may have stripped it of points for the current UY and borrowed all of the points from the next UY or, on the other extreme, there may be banked points from the previous UY, all current UY points and all future points intact (a "fully loaded" contract).
Yeah, you are right, the resale on BWI is the way to go for that resort.
Just an FYI: BWI is the hotel side of the resort, BWV is the DVC side of the resort.