Remember, that if you hold a current and valid medical license, you can treat ANY patient for ANYTHING, from Derm to Thoracic surgery. However, if you plan to get hospital privileges, get insurance reimbursements and not spend most of your time in courtrooms being sued by patients, you'll need a board certification issued by a board which is a member of the ABMS.
So, if you want to part with your money, you can decorate your office with fancy diplomas in "cosmetic surgery", "aestetic dermatology" and other such nonsense to all your heart's content. But the bottom line is, that it will NOT prove that you have qualifications to treat patients at at a professional level, as compared with "real" board certified physicians.