Two things stop it: the scale required, and the fact that rigged sales never count as comps. Moving an actual market would take an enormous number of sales and an enormous amount of capital, far beyond a below-market deal here and there. More important, appraisers and lenders only use arm's-length, open-market transactions to establish value. A sale to yourself, a family member, a friend, or a business partner is a related-party transaction, and assessors and appraisers specifically exclude those. You can see the safeguard in ordinary practice. Buy a property off-market at a low price and the county assessor and the appraiser do not automatically treat that price as market value. You often have to submit photos and documentation showing why the price was low (poor condition, a distressed seller, an insider deal) just to get the value recognized at all. So a rigged low sale does not drag the neighborhood down, because the people who set values throw it out. The system is built to look through exactly this kind of manipulation.