I'm the co-founder of www.crated.com a marketplace for photographers and digital artists.

I'm going to say SUPPLY. Simple reason if your buyers land on your site and don't find anything you're done. They won't come back. Also the supply side users can be instrumental in helping promote and drive traffic to your site in the early days by putting them to work.

I used Clarity to speak with Micha Kaufman founder of Fiverr and he's a big reason for us shifting our strategy to building the supply side well in advance of launching our site which is currently in beta. He strongly suggested building SUPPLY.

Really surprised to see Dan Martel saying DEMAND since I know he worked hard to get the first 1000 experts on board before launching. Sorry to call you out Dan :-)

Supply! Supply! Supply!

IMHO.

Adrian :-)
Co-founder
CanvasPop / DNA11 / CRATED