This is a solved problem. When you sell a house in the US, you ahve to determine what your capital gain is for tax purposes. That includes all purchase and selling costs (eg agents fees, transfer taxes, etc). Those are all added to the purchase price to determine your cost basis.

The capital gain is simply the sale price minus the cost basis, which might be a loss.

So if you've paid unrealized capital gains taxes along the way, you either get credit for those taxes already paid (and possibly get a refund if you've overpaid) or they're simply added to the cost basis.