To expand a bit further on j45's points, this sounds like it's the OP's first time doing this. If there are enough resources to seriously investigate if this reseller model could be scaled to different ad-ops agencies/brokers, then it makes sense to go with a trial period.
If the deal goes nowhere, then at least you'll have a data point to figure out if you want to pursue this model further (as opposed to a hypothetical, educated guess).
One thing to consider is making sure that the reseller uses the same licensing/payment model on both sides of the contract. This is mainly to avoid a potential issue where the reseller gambles by going with a (much higher) fixed price site/unlimited license model on the end-user* side, while keeping the utility model on your side. This would only work if the reseller believes that they can accurately forecast maximum end-user volume.
I'm aware of situations where the licensing model was not mirrored between reseller and software OEM, resulting in awkward contract discussions between the upstream parties.
This can be somewhat (but not entirely) mitigated with high-watermark volume triggers, and (most important!) an agreed-upon volume metric used in the licensing discussions. You may think that the latter is easy enough to rely on since you're the one serving the ads on-demand, but how do you know you're serving the ads directly to the end-user and not going through a/the reseller's aggregation platform instead?
*Note: "End-user" refers to the customer paying for the ads to be displayed, not browsing audience.