How pays for the HST on a consignment - buyer 1 or buyer 2 ?

Depends on the agreement, if you are , check the paperwork to see what it says. Typically HST is included in the final price unless stated otherwise.

If you are wondering if there is a standard, everything is negotiable, but I would imagine Buyer #1 wants to walk away free and clear, so if they are marking up the property I would argue they can take the HST from their profit, but if they are assigning it at cost why would they want to absorb the cost, Buyer #2 can pay it or Buyer #1 can wait until the property closes and then just sell it for a profit.

Make sense?

Since HST is only payable at registration it would seem only logical buyer 2 would be responsible

As far as I understand; if buyer 1 bought the property as an investment the builder is not going to get a credit on this unit and therefore buyer 1 is responsible for the HST.
Consider it never closed and buyer #2 Buys the Agreement of Purchase and Sale, which is a contract between buyer 1 and builder and therefore is liable to the same conditions as buyer #1.
Conclusion buyer #2 pays the HST.