I have 300 000 Delta reward miles and don't need tickets from my Airline.
Where can I sell there airmiles?
If it's not possible to sell them, where can I exchange them for miles of other airlines?
The majority of airline frequent flyer programs prohibit the sale of miles. You can donate them to certain causes, you can book tickets for your family and friends, you can use them to book hotels through their vacation programs, you can sometimes convert them for other programs. But you can't sell them for cash.
There are some agencies that offer to "buy miles" but essentially what they do is have you book a ticket for someone using your points, which they then sell and share the profits with you. BUT if the airline finds out (and they often do) the airline will bill the buyer for the full cost of the tickets and they will confiscate what you have left in your FF account and drop you from the program. A fair number of the "cheap business class tickets" websites provide FF exchange tickets and the web is littered with complaints and warnings from folks who found out the hard way that the airlines don't allow such.