What I've typically seen in all my years in the hobby is that when people are willing to pay finders fees it because they want something so specific that they can not easily find it themselves. Think the 1 of 20 type cars with very specific option lists, colors, and/or drivetrains.
If a buyer has some project space, time, and money and are not too particular, they probably are not going to seek out specific details of a car much more than the make, model, and year range and probably will not be in touch with services like this.
I'd also second what was just said about dealer laws and encourage you to do some research on that. Some areas of the country can get real specific about the regulations governing auto brokerage, which is what you would be doing. Also worth noting if your going to buy the cars yourselves and then resell them, is that then your crossing the line in to being a dealer or wholesaler, which gets you in to a whole new set of rules.