This may already be supported somehow, or otherwise planned, but one obvious way to monetize might be to act as a broker for contract designers. As a user, if I search for an icon and either don't find one that works for me, or find only icons with incompatible license terms, then the next thing I'm going to want to do is find someone who can design what I want. This is also going to be true in the event that I need a whole set of icons with a common motif or color/design scheme.

No matter how good a search engine like yours is, most of its users will be at least partially unsatisfied, and you may be able to turn that into an advantage. It might also be a good way to stand out from the copycat sites.