I think you should learn how the markets work. What you have described above is an OTC (Over-The-Counter) transaction which is not on a public exchange, but offline or on an exchange like LocalBitcoins. The market cap of an asset is determined by the product of the last market price and the number of units in circulation. Moreover you're just transfering your asset to another person, you're not really taking away from MC. You can only do that if you have the financial power to change prices like making a huge market order of tens of million of dollars worth depending on the liquidity of the market and the participants.