That's not how US capital gains tax law works. It's legal to sell something at below market value, but you have to use the fair market value when calculating a loss for tax purposes. Of course some people cheat.
https://www.irs.gov/taxtopics/tc409
> When you sell a capital asset, the difference between the adjusted basis in the asset and the amount you realized from the sale is a capital gain or a capital loss.
Am I misinterpreting this?