Since you where collecting sign-ups, it would be really interesting to see registrations by marketing channel and calculate a CPA for each. In terms of building the "seller" or in this case, the "lens renter" side of the market, I'd probably do targeted facebook ads to photographers with messaging around "Make Money Renting Your Gear" or even more specific messaging "Make up to $20/day/lens renting your unused gear".

You could also try posting to various email lists/meetup groups/forums where photographers hang out asking for beta testers with whom you take 0% commission. Most likely, this is where you'll see the most success.

You could also go for the Chegg/Zipcar model, which obviously has much higher capital costs, where you own the inventory.

PPC could be a viable customer acquisition channel for you as well as "camera lens rental + city name" has decent volume, as does "lens name + rental" and "lens name + rental + city", so there's a long tail that you can exploit.

All that being said, there is quite a bit of competition in this market and it will be difficult to differentiate:

http://www.borrowlenses.com/

http://www.cameralensrentals.com/

http://www.lensrentals.com/

http://lensrentalscanada.com/

http://www.lenslenders.ca/

P.S. Glad to hear that 99designs ended up driving some good traffic to your site... I've also seen people list websites on Flippa.com (often with incredibly high prices) as a form of a growth hack when trying to reach internet marketers/affiliates, though we strongly discourage it.