there is a site called Fiverr, it does a similar thing, though obviously you'd have to work around the lewd aspect and just pretend they were "concept sketches" and whatnot.
The way it works is, a writer/artist/whatever creates "gigs" and then you go in and select the option you want, pay the money and it generates a timer based on how quickly the creator has listed that they'll do shit.
Once they've finished, they hit the "delivered" button and it issues a confirmation to you. you then hit your button and it all goes through.
I dunno if i explained that right, but it may be worth a look into by some people.
pic related it something i just finished sketching up this morning.