Airlines generally do not transfer ownership of those miles - that subjects you to their conditions (and it also means you don't pay IRS or state taxes on those miles or their use).

Selling miles is usually viewed as fraud (violation of the terms and conditions of the airline) and some airlines are particularly aggressive about prosecuting this - cancelling miles, even status and account; those flying on purchased, bartered or traded awards may be denied boarding unless they pay full "walk-up" fares.