You're right David but, as someone who has built a couple of marketplaces, I find that founders often don't spend enough time on the "tougher" side. The common

You're right David but, as someone who has built a couple of marketplaces, I find that founders often don't spend enough time on the "tougher" side. The common mistake (been there, done that) is to spend something like 60% of your time building the harder side and 40% of your time building the easier side. Instead, you should probably spend something more like 98% of your time building the harder side and 2% of your time building the easier side. Which is the harder side? Generally, it's the people with the money. :) However that's not always the case. If you've stumbled into a massive market with high demonstrated spend it might actually be harder to build the supply side. I'd side that's likely the case with a company like AirBnB (they've done a great job building both sides of course). If you can build the more difficult side (usually demand) it will typically pull supply out. It typically doesn't work the same in the opposite direction.