I thought about doing this, for my daughter who wanted to buy a house in our neighbourhood to live in. If you're looking to flip, I'm not sure a personalized letter will come across any differently than the infamous yellow letters. So Walt's idea of a visit in person might be better. OTOH, I know some elderly people who'd be VERY reluctant to open the door to a stranger, especially of that stranger were a strong-looking young man.
My daughter, btw, bought a fixer-upper before I wrote any letters. She and her husband did the minimum to make it habitable, then moved in while they slowly fix up the rest. They've gotten several letters offering to buy their "abandoned property."