What is the problem of trust between advertising buyers and sellers?
At first, you should make it clear, that advertising buyers here are advertisers, and sellers ads platform publishers.
The essence of the problem is that buyers often waste time on selecting the right ads platform and suitable publishers. Moreover, they are unable to evaluate the effectiveness and quality of advertising before actual implementation. This leads to uncertainty about the level of efficiency and whether the KPIs promise by the sellers will be met.
For sellers, there is a risk of non-payment, as they generally provide the service before payment is completed. For instance, Google, Facebook and similar platforms hosting ads are forced to lock their customers accounts, due to them not having enough funds to cover transactions on due date. This may results in these customers being in debt to the platform, with debt collecting fees negatively influencing the platforms profit margin.