Notice of default filings are public and free, and most buyers treat them as a lead list without asking what the list actually measures.
A notice of default tells you the borrower missed payments and the lender filed. It tells you nothing about equity, nothing about whether the owner is reachable, and nothing about whether three other buyers sent mail last Tuesday. In a county that records fast, a filing from two weeks ago might already have a cure in process or a listing agreement sitting on the kitchen table. The filing date is a timestamp on a financial event, not an invitation. What makes a name worth working is equity first, then recency, then whether the property has other encumbrances that would eat the spread before you got to it. Take a house with a 400k value, a 310k first, and a 22k second from 2007 that never got released. Your margin math on the first looks fine until title comes back and the second is still open, and now you are negotiating a payoff on a lien the seller did not know existed, against an auction date that does not care. The filing gave you none of that. The filing gave you a name and an address.
The sourcing question worth asking is whether the county records are detailed enough to filter by loan origination year and approximate balance before you spend anything on outreach. Some recording offices show the original mortgage amount and date on the public instrument, which lets you back into a rough equity estimate before the first stamp goes on an envelope. Others show almost nothing and you are mailing blind into a list that is half underwater, half already in reinstatement, and a small fraction actually worth a conversation. What does your county's recording data actually surface on the original mortgage instrument?