A lot of the talk has been about the value of marketing vs selling etc, but you asked about how to quantify performance and not get screwed over, so I'm going to try a coherent answer. Short version is: it's hard to quantify, for a variety of reasons. I've consulted on a few projects lately where marketing was brought in to push the product to the masses, with varying results. This is what I've learned:
First, you want someone with experience in the same market niche your product is aimed at. I was in a meeting with a marketer who said he was a perfect fit because he did mobile, though it was games for preschoolers, and we were doing enterprise software. Bad fit. That much is obvious, maybe, but then you'll find the marketer who hits very close to your niche, and with tangible success, and you'll hear about the tens of thousands of sales he got in his first month at his last gig, and you'll start seeing dollar signs floating everywhere... and that is a bad thing.
Before you talk to a marketer, do some work and figure out what your "happy" outlook is like. Never mind units, focus on profit, because when the money starts coming in, the units become irrelevant and all you'll care about is how close you are to breaking even. If you've got an expiry date on your endeavour (the point at which you have to get another job to pay the rent), figure out how long it'll take you to get there, and then merge it with your "happy" outlook and make that your benchmark. "In six months, we need to earn $10,000." Simple, bloody-minded, realistic. Do this before you talk to any marketers, because it will be hard to be honest with yourself after.
A marketer will ask you what your sales goals are, and no matter what you answer, you'll end up with a number based on their market analysis and track record. This is not necessarily a bad thing, but you have to keep it separate from your own analysis of reality. Once you've heard from all your candidates, pick the one who actually listened to you when you spoke. A lot of them don't, and no matter how brilliant they may be, if they're not listening to you, you will end up hating them very quickly. You can't hate your team. At least not right away.
At this point, you've got a best-guess marketer, with a half-decent chance of success. Set parameters: they have X months to deliver Y results. If you're cautious, set it at some fraction of your expiry date. Another good approach is to just say: "Look, you have six months to earn us $10,000, or we're toast." Some people react well to knowing where the cliff lies, some people freak out and leave. But setting a concrete goal gives them focus, and something to beat. In their contract, make that key: you fail at this, you're out.
That said, you are going to spend 90% of that time period thinking you made a terrible mistake. The more time you spend talking to marketers, the more you can see the smoke and mirrors and fishing wire they use to do their thing, and the more it scares the crap out of you. Wait, we're trying to sell this thing based on anticipation and close-up photos of flowers? Seriously? Do I really have to post that to Facebook? I won't have any friends left by the time the product comes out...
Unless the marketer is actively drinking scotch in meetings, you've got to let them do their thing. Because — and this is the sucky part — you're going over that cliff one way or another. If you chose wisely, your marketer will have built a set of wings to glide over the canyon, and you'll be fine. And if not... well, at least you get to go splat with pizzaz.
