Of the old ad networks only doubleclick survives and they only do so because of google massive anti-fraud measures.
Anybody that gets into ads these days will have to take abuse as their #1 priority to be able to compete so if this is to work at all you can bet that anti-fraud measures would make up a very large component of the package.
Tying the end-user and one of the primary beneficiaries to the same chair is a risky thing but not all forms of advertising are equally susceptible to this kind of risk. For instance, performance based advertising and to a lesser extent branding are not all that much at risk.
On another note fraud is present in all forms of online advertising at the moment (with the same qualifications listed above) and it is simply priced into the ad rates.