I've not had the occasion to use SoundCloud just yet, so I don't know if I can upload MIDI files. Can anyone advise me on this issue.  I want to post them to SoundCloud, so I can sell them on another website.
Thanks,
KC

You will be better off using DROPBOX and the X3 included Gobbler.
SoundCloud deals with digital audio as a specialty and may not deal with the whole world of possible every computer needs. They cannot even do video, like YouTube etc. 

you'll probably get more help / answers by putting this thread where it belongs.
this really has little to do with sonar X Series.
 
its a midi / web question.

There are dozens and dozens of ways to sell stuff on the internet. Soundcloud is not what one would use for MIDI or a good Kitchen. It is for streaming and downloading audio in an MP3 format.
But you could convert your MIDI files to audio and post them there as examples for people to pre view. 
In a way that's safer as you midi files themselves would be there for someone to hack. 
Your own web site will give you the highest profit and control. 
Then there are sites made for selling, like E bay. 
The likely hood of reaching a market is where you'll need to be prepared to learn the ropes, there are a million freebies out there.