This is an easy question to answer for you. The $500,000 sale price recorded in May was from a Trustee Sale where the first position lender/bank took back the home from the previous owner due to a foreclosure.
At a Trustee Sale a home is offered up for auction by the bank for it is the last step of the "Notice of Default" process here in the state of California. The starting auction bid for any home which is a NOD will include: Loan amount due, attorney fees, foreclosure cost, late and penalty fees and other miscellaneous cost. And this is why the majority, if not all, of the homes at a Trustee Sale go unsold.
The county in which the home is located in will record a deed showing the new owner of record for legal purposes even though the home did not officially sell to anyone.
Sometimes the tax record will show as a "sale price" what the bank may have taken a property back for (probably the amount of roughly the 1st lien) & it appears that there was actually a Real Market Sale, when there really wasn't. Sites like Zillow may show it as a market sale for May 2009.
Is this property back on the market as a bank owned sale for $305k?
The public records system occasionally reports incorrect information.