I recently had been contemplating selling material to the Music Bakery and even met with some of their people to get reactions on what material from my own catalogue of unsold pieces would appeal to these corporate types. As it turned out they loved my VSL stuff but they really want tracks that sound like the evening news; high energy orchestra w/ rock rhythm section.
Dallas is big in the corporate music scene and friends of mine have done a lot of this kind of thing, but I never had the stomach to put together that stuff at the production level that gets the good dollar (and EVERYTHING is a buyout--no royalties). Besides, they want stems for everything. Every drum track soloed, etc. so they can remix and exploit one track into a bunch of variations, thus generating many times the material from the music submitted.
The Bakery also will not accept filmic loops like from Stylus or Distorted reality. They will do all of that. They want real drums & perc., real brass, etc. Skillful mixing can get VSL to sound great but since they will be remixing everything the tracks have to sound great soloed. Going through all this trouble for music I couldn't care less for makes the situation unworkable for me.
The Fat Man in Austin can act as a brokerage for music as well. If, however, you look at what they'll pay you then look at what they charge their clients you probably wouldn't consider library music. You can even download a contract from their website to see how some of that stuff works out on paper. They don't actually buy tracks, they are given exclusive rights for licensing your music and they give you like %5 of what they make or something.
If you can find a library that will pay out decent royalties, etc., then (i believe) you will have found a rare service. All that I have encountered (save for the Fat Man) require a buyout.
Clark