It is a very good concept - getting logos is typically very expensive and time consuming. This makes it quick, easy and inexpensive.
It would be good to have a try before you buy option, so the user can see how the site works (how easy it is to use - many people using it will not be very computer literate, so they might be put off if they think the site is difficult to use). The try before you buy option could show a few sample logos, with 'sample' written across them and not allow you to download them.