The key would be to have the bad bank receive it's assets at discounts that would already be attractive to the private sector.  50% may not be low enough for private foreign capital to buy.  The ideal scenario would be for the bad bank of buy the assets at a 75% discount and sell the assets to private investors in chunks.  The profit made by the bad bank would be passed on to Spanish taxpayers.

From Reuters

Spain bad bank assets to get 45-50 pct avg discount-sources

MADRID, Sept 24 | Mon Sep 24, 2012 1:20pm BST

(Reuters) - Lenders will transfer property assets into Spain's new bad bank at an average discount of 45 percent to 50 percent of original book value, three banking sources said.

The figure is obtained by applying an additional discount of 5 percent to 10 percent over the average writedowns of around 40 percent that the government has already forced banks to take on real estate assets, the sources said.

"As an average we could see further writedowns of 10 percent," said one of the sources - a manager at a Spanish bank.

A second source said the additional discount would be equivalent to a capital buffer lenders were asked to set aside earlier this year but which has now been made irrelevant by new, higher capital requirements.

By assuring that the new writedowns would be compensated by the capital buffer, Spanish authorities would protect banks from booking new losses.

However, the discounts will likely not be steep enough to attract foreign investors to the bad bank. That could eventually force the state to use more taxpayers' money to take over the toxic assets to later sell them off.

The sources cautioned that the discount was still being discussed and could vary depending on the type of assets that will be transferred, a key question that has yet to be answered.

The Bank of Spain and the European Commission, which are leading the negotiations and will have the final say on the pricing of the assets, declined to comment on the matter.