A postcard will get better results than a letter! It does not need to be opened — the seller will see your message right away.
You should keep it fairy generic, as Don said.
My postcards said something like
Sell Your House In 7 Days
Any Condition
No Real Estate Commission
No Closing Costs
Take what you want, leave everything else at the house
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I’m out of the country now so I do not have access to my old files. When I get back home, I will upload my postcards and other marketing materials to the File Vault at
https://www.cashflowdepot.com/education/file-vault/