Garageband for microstock

I primarily work in video and photos but also have the ability to produce music. I occasionally license sound effects but wanted to license music for either adding to my own videos for sale or just license the music. I have a MIDI controller and use garageband's software instrument. I just wanted to verify that using the built in software instruments is ok for stock sales, I would obviously not use any of the pre composed loops just the software instruments.

If you can produce some commercial sounding tracks in GarageBand - go for it.

Yes you can. You have a lot of good instruments in garage band. To start it is correct.