Haveing a flipper or two around means that sellers are harder for buyers to cheat.
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I really question that. At the end of the housing bubble, nearly all sales were to flippers, and most of them were recruited by creeps who told them of the great wealth they were going to make for doing essentially nothing. And said flippers got stuck holding the bag, with no one to buy their overpriced houses, no money to fix up said houses, no one who could pay enough rent to cover their payments. THEN they lost the houses and they sat empty. It served the stupid flippers right.
Flippers who do not fix up the property are not only useless, they simply serve to raise the price of houses.