I never approached anybody to buy my shots. I always get people / firms emailing me a link to my photograph either here on Airliners.net or my Flickr gallery page. Sometimes, its difficult to get hold of an old shot which someone is interested in buying and is lost somewhere in my hard drive. Considering the amount of people that go through Airliners.net, whatever shots I have sold up till now are due to A.net. Very few due to Flickr, honestly.
One way to go about is it to advertise about your aviation photography, try and spread the word. What my experience has been with these people is that they usually go by contacting two or three people who have a shot of the aircraft that they are looking for. And then they possibly choose the one who is ready to give it to them for the lowest price.
For me, it has always been just out of the blue emails, suddenly appearing from nowhere.
All the best.
[Edited 2013-03-06 17:59:06]
"Caution, Wake Turbulence"