Unluckily ads are extremely intrusive.. and a large number of people go out of their way to find ways to block them.
A better solution, if the niche is one that would support it, is having some vendors onboarded and give them a special support/sales area to work from as well as having a feature where they could list "special items" at "reduced" prices to the members. In my niche, the markup on most stuff is such that a regular 5% off if purchased from the site wouldn't not hurt them, and if the site got percentage of the sales through that process, it could be a steady income stream.