(This post wound up being tl;dr. Sorry.)
Here's a quick analogy.
You sound like you are a technical person, such as a developer.
Suppose an "idea man" contacted you and told you that they wanted you to contribute your talents to their venture without up front money, and for you to only get paid when the venture starts to return revenues. So this "idea man" who claims to have sales and marketing expertise is saying that he has a "sure thing" and only needs your contribution.
He wants you to spend X months writing a program that he claims that he will sell, and this will make both of you money.
And you don't know this person at all right now. He contacts you out of the blue. You have to trust that he has a great idea. That he will be fair about rewarding your efforts. That you have no better opportunity at hand.
This is exactly what you are asking, except turned around. I placed you in the role of the "hypothetical partner".
EVERY startup I have ever spoken with wants exactly what you want: assured results from engaging with an outsider, with no investment or risk the startup's part.
Not going to happen.
As a reality check, Google for "copywriter" - a type of marketing expert - look at the top copywriter's web sites, and see if any of them will work only for a share of the profits or will guarantee their work.
This has symmetry with the OP's contention that he does not want to pay a "slick talking sales type" lots of money with no results. From the marketer's point of view, the OP lacks a track record of selling a product or service successfully so on the other side he is unclear whether his time will be wasted.
Only a hideously inexperienced marketer (IE: one who really isn't a marketer) would walk into this trap. Or someone who tells the OP exactly what he wants to hear.
There is also a logical contradiction in what the original poster wants. The contradiction is that the original poster wants guaranteed results, which implies that he is hiring a top marketing person, but anyone with the ability to guarantee results will not be available at the scale or money that the OP is able to pay.
There is a much better way to go forward, and it involves discarding the OP's assumptions about sinister greedy sales people who are out to con everyone, as well as the black and white thinking about assuming that there is either success or failure, switched on or off like a light switch.
The bottom line is this: You should expect to pay anyone decent, and anyone decent isn't interested in gambling in order to get paid. Competent, proven people don't work for free or on speculations. And your business as it now stands is a gamble to everyone. You are bringing little to the table.
Secondly, you should expect to pay for a LITTLE advice - a review of your site, your materials, your entire business plan - from someone with a demonstrable track record in developing new business.
I develop copy for ISVs and I find that those starting out have very fundamental BUT relatively straightforward to correct flaws in their premise or their copy.
It often comes down to something very obvious (to an outsider) like unreadable copy that is written by a programmer that contains no bullet points or titles, or the absolute absence of benefits and the overwhelming of the reader with technical details. These, by the way, are the most common problems with all newbie copy.
I'm saying that your current problems with your offer are probably relatively obvious to the right eyes. It should not cost much even for an experienced copywriter or marketer to make a "first order" pass through everything you're doing now and tell you what needs to be corrected.
Or maybe it's not even a copy problem. Maybe your search engine standing is poor. Or maybe you can't market your type of service by the internet and instead you need to market directly to users. I tell my clients, don't expect a $5000 B2B product to sell itself through a web site right out of the gate.
Another thing you should do is discard the thinking that a really effective marketer is going to wave a magic wand with no participation from you, and that they will disappear for months and cost you six digits.
No client allows anyone to work like that. Why would you ever let them?
You should know if the work is on-track simply because you will see someone else's work on a daily or weekly basis and you will react to it as a new prospect would. You look for revisions and work that make sense to you and which seem to correct problems.
If you think the work is shoddy or stupid, you stop it early and you fire the person and you find someone else.
One last thing. If you develop an ongoing relationship with a competent marketer, you should experience a bit of internal transformation yourself. You should start to see the things that the marketer sees. Eventually, every small business owner needs to be their own marketer, and you will come to rely on the marketer less and less.
But you need active mentorship from an experienced professional in order for this to happen.