I am a newspaper photographer that also freelances commercial work and has a stock agent that handles my stock images.
Stock agents (picture libraries) want transparencies as do the magazines with whom I have dealt. Agents want LOTS of images to keep on file for potential sales. Usually they are not interested in the hobby photograher with a few images.
As for selling/marketing your own work, go for it. But if you do, charge the going rate for whatever that picture would normally cost from a pro.
What happens many times is the hobby photographer lets his/her images be used for a credit line or for really cheap.
That has consequences. It takes money out of the pro photographers pocket and magazines/publications start thinking that photos are basically worthless and why hire a pro or buy from a pro if someone who's work is almost as good gives it away.
Too many times editors have said that they can get pics cheap so why pay. Weird...they pay writers for their words. they pay the guy that sells them the paper for their magazine, they pay for ink, but they want images for free. The perception is that anyone can take a photo.
True, anyone can TAKE a photo, but not everyone can MAKE a saleable image.
Amateur shooters are thrilled to get a buyline...but let me tell you, buylines do NOT put food on the table, nor film in the camera nor shoes for the kiddies. Those are all things that a pro shooter counts on by selling work.
We have a great example here where I live: The local minor league baseball team used to pay one of the local pro shooters to photograph the players, games etc.
Along comes a talented amateur and does it all for a couple of season passes. After all, he is just doing it for fun and not for a living. He has a "real" job" so he doesn't need to get paid for the pics.
His work is not as good as the pros but that's ok, it is good enough.
The quality is less, BUT most importantly the perceived value of photos is diminished.
And the pro guy? Well he still has to have several thousand dollars of equimpment (film and now digital) to compete in the market place, insurance for the gear, and all the other things we need to work and make a living. And he has one less client because someone else did not value his work enough to get paid for it.
So, if you sell your work, don't undercut the going rate. If it is worth using it is worth paying for.
My 2ΒΆ worth