As some of you may have noticed, I do a lot of GA shots - largely because the photo opportunities are better on small private fields - but it is also a fact that I've sold more pics in this category than any other, despite them being relatively "low hitters". Most sales are, I believe to the pilots and passengers, though a good few are to various GA service industries.
So the answer is, yes there is a market. In my experience, you need shots which have a bit of drama - standard 50mm side views don't seem to sell (I guess the owner can see this any time!) - but a shot with the owner/pilot in action may have a chance. If you want to do this on a regular basis, I think the most important thing to keep in mind is that you're photographing someone's "pride 'n' joy" - make the plane look good and the shot interesting - which is a real challenge with your average spam can! ... and try and avoid shots where the pilot has his finger up his nose
How best to market these types of pics is another matter - not every GA pilot visits A.net (and I want to know why not!), so if you want to sell seriously, you'll have to take the pics to the pilots. I have considered setting up a stall at various fly-ins and the like, but frankly, I'd much rather be out and about behind a camera than sitting behind the till - but if you're the enterprising type, I'm sure there is a healthy market to be exploited here, esp. if you're equipped to produce prints on demand of attending aircraft.
Colin K. Work, Pixstel