> Surely each offer round becomes a bidding round, rather than a simple purchase.
No:
1. There's no requirement to list the house - who would even know if you sold it twice a year?
2. There's no requirement that the seller has to take the highest offer. The other bidder will have to bid again next year when taxes are due.[1]
[1] You can't set the tax based on what the bid is, because the bidder may not have the money (or may not be granted the bond) when time comes to actually do the transfer. The tax has to be based on what the actual selling price is.